title: Adding radius slider to Google Maps JavaScript API
body: <p>I'm getting some weird behavior when I try to dynamically change the radius in a heatmap made with the Google Maps JavaScript API.  I've tried to add a slider to the HTML so that the radius can be selected by the user.  I've done the same for opacity, and it worked beautifully.  When I adjust the radius slider, however, odd things happen.  It only adjusts in one tile of the map, and when I zoom, the entire heatmap overlay disappears forever.  (Zoom behavior gets pretty screwy too.)  As a simple example of this behavior, I've put the code from the <a href="https://developers.google.com/maps/documentation/javascript/examples/layer-heatmap" rel="nofollow">Google Maps JavaScript API heatmap example</a> below, with the only changes to the code being to implement sliders for opacity and radius.  Can you see where my problem is?  My only changes are at the very bottom of the code, and I've commented them to help you see.</p>
  
  <pre><code>&lt;!DOCTYPE html&gt;
  &lt;html&gt;
    &lt;head&gt;
      &lt;meta charset="utf-8"&gt;
      &lt;title&gt;Heatmaps&lt;/title&gt;
      &lt;style&gt;
        html, body, #map-canvas {
          height: 100%;
          margin: 0px;
          padding: 0px
        }
        #panel {
          position: absolute;
          top: 5px;
          left: 50%;
          margin-left: -180px;
          z-index: 5;
          background-color: #fff;
          padding: 5px;
          border: 1px solid #999;
        }
      &lt;/style&gt;
      &lt;script src="https://maps.googleapis.com/maps/api/js?v=3.exp&amp;sensor=false&amp;libraries=visualization"&gt;&lt;/script&gt;
      &lt;script&gt;
  // Adding 500 Data Points
  var map, pointarray, heatmap;
  
  var taxiData = [
    new google.maps.LatLng(37.782551, -122.445368),
    new google.maps.LatLng(37.782745, -122.444586),
    new google.maps.LatLng(37.782842, -122.443688),
    new google.maps.LatLng(37.782919, -122.442815),
    new google.maps.LatLng(37.782992, -122.442112),
    new google.maps.LatLng(37.783100, -122.441461),
    new google.maps.LatLng(37.783206, -122.440829),
    new google.maps.LatLng(37.783273, -122.440324),
    new google.maps.LatLng(37.783316, -122.440023),
    new google.maps.LatLng(37.783357, -122.439794),
    new google.maps.LatLng(37.783371, -122.439687),
    new google.maps.LatLng(37.783368, -122.439666),
    new google.maps.LatLng(37.783383, -122.439594),
    new google.maps.LatLng(37.783508, -122.439525),
    new google.maps.LatLng(37.783842, -122.439591),
    new google.maps.LatLng(37.784147, -122.439668),
    new google.maps.LatLng(37.784206, -122.439686),
    new google.maps.LatLng(37.784386, -122.439790),
    new google.maps.LatLng(37.784701, -122.439902),
    new google.maps.LatLng(37.784965, -122.439938),
    new google.maps.LatLng(37.785010, -122.439947),
    new google.maps.LatLng(37.785360, -122.439952),
    new google.maps.LatLng(37.785715, -122.440030),
    new google.maps.LatLng(37.786117, -122.440119),
    new google.maps.LatLng(37.786564, -122.440209),
    new google.maps.LatLng(37.786905, -122.440270),
    new google.maps.LatLng(37.786956, -122.440279),
    new google.maps.LatLng(37.800224, -122.433520),
    new google.maps.LatLng(37.800155, -122.434101),
    new google.maps.LatLng(37.800160, -122.434430),
    new google.maps.LatLng(37.800378, -122.434527),
    new google.maps.LatLng(37.800738, -122.434598),
    new google.maps.LatLng(37.800938, -122.434650),
    new google.maps.LatLng(37.801024, -122.434889),
    new google.maps.LatLng(37.800955, -122.435392),
    new google.maps.LatLng(37.800886, -122.435959),
    new google.maps.LatLng(37.800811, -122.436275),
    new google.maps.LatLng(37.800788, -122.436299),
    new google.maps.LatLng(37.800719, -122.436302),
    new google.maps.LatLng(37.800702, -122.436298),
    new google.maps.LatLng(37.800661, -122.436273),
    new google.maps.LatLng(37.800395, -122.436172),
    new google.maps.LatLng(37.800228, -122.436116),
    new google.maps.LatLng(37.800169, -122.436130),
    new google.maps.LatLng(37.800066, -122.436167),
    new google.maps.LatLng(37.784345, -122.422922),
    new google.maps.LatLng(37.784389, -122.422926),
    new google.maps.LatLng(37.784437, -122.422924),
    new google.maps.LatLng(37.784746, -122.422818),
    new google.maps.LatLng(37.785436, -122.422959),
    new google.maps.LatLng(37.786120, -122.423112),
    new google.maps.LatLng(37.786433, -122.423029),
    new google.maps.LatLng(37.786631, -122.421213),
    new google.maps.LatLng(37.786660, -122.421033),
    new google.maps.LatLng(37.786801, -122.420141),
    new google.maps.LatLng(37.786823, -122.420034),
    new google.maps.LatLng(37.786831, -122.419916),
    new google.maps.LatLng(37.787034, -122.418208),
    new google.maps.LatLng(37.787056, -122.418034),
    new google.maps.LatLng(37.787169, -122.417145),
    new google.maps.LatLng(37.787217, -122.416715),
    new google.maps.LatLng(37.786144, -122.416403),
    new google.maps.LatLng(37.785292, -122.416257),
    new google.maps.LatLng(37.780666, -122.390374),
    new google.maps.LatLng(37.780501, -122.391281),
    new google.maps.LatLng(37.780148, -122.392052),
    new google.maps.LatLng(37.780173, -122.391148),
    new google.maps.LatLng(37.780693, -122.390592),
    new google.maps.LatLng(37.781261, -122.391142),
    new google.maps.LatLng(37.781808, -122.391730),
    new google.maps.LatLng(37.782340, -122.392341),
    new google.maps.LatLng(37.782812, -122.393022),
    new google.maps.LatLng(37.783300, -122.393672),
    new google.maps.LatLng(37.783809, -122.394275),
    new google.maps.LatLng(37.784246, -122.394979),
    new google.maps.LatLng(37.784791, -122.395958),
    new google.maps.LatLng(37.785675, -122.396746),
    new google.maps.LatLng(37.786262, -122.395780),
    new google.maps.LatLng(37.786776, -122.395093),
    new google.maps.LatLng(37.787282, -122.394426),
    new google.maps.LatLng(37.787783, -122.393767),
    new google.maps.LatLng(37.788343, -122.393184),
    new google.maps.LatLng(37.788895, -122.392506),
    new google.maps.LatLng(37.789371, -122.391701),
    new google.maps.LatLng(37.789722, -122.390952),
    new google.maps.LatLng(37.790315, -122.390305),
    new google.maps.LatLng(37.790738, -122.389616),
    new google.maps.LatLng(37.779448, -122.438702),
    new google.maps.LatLng(37.779023, -122.438585),
    new google.maps.LatLng(37.778542, -122.438492),
    new google.maps.LatLng(37.778100, -122.438411),
    new google.maps.LatLng(37.777986, -122.438376),
    new google.maps.LatLng(37.777680, -122.438313),
    new google.maps.LatLng(37.777316, -122.438273),
    new google.maps.LatLng(37.777135, -122.438254),
    new google.maps.LatLng(37.776987, -122.438303),
    new google.maps.LatLng(37.776946, -122.438404),
    new google.maps.LatLng(37.776944, -122.438467),
    new google.maps.LatLng(37.776892, -122.438459),
    new google.maps.LatLng(37.776842, -122.438442),
    new google.maps.LatLng(37.776822, -122.438391),
    new google.maps.LatLng(37.776814, -122.438412),
    new google.maps.LatLng(37.776787, -122.438628),
    new google.maps.LatLng(37.776729, -122.438650),
    new google.maps.LatLng(37.776759, -122.438677),
    new google.maps.LatLng(37.776772, -122.438498),
    new google.maps.LatLng(37.776787, -122.438389),
    new google.maps.LatLng(37.776848, -122.438283),
    new google.maps.LatLng(37.776870, -122.438239),
    new google.maps.LatLng(37.777015, -122.438198),
    new google.maps.LatLng(37.777333, -122.438256),
    new google.maps.LatLng(37.777595, -122.438308),
    new google.maps.LatLng(37.777797, -122.438344),
    new google.maps.LatLng(37.778160, -122.438442),
    new google.maps.LatLng(37.778414, -122.438508),
    new google.maps.LatLng(37.778445, -122.438516),
    new google.maps.LatLng(37.778503, -122.438529),
    new google.maps.LatLng(37.778607, -122.438549),
    new google.maps.LatLng(37.778670, -122.438644),
    new google.maps.LatLng(37.778847, -122.438706),
    new google.maps.LatLng(37.779240, -122.438744),
    new google.maps.LatLng(37.779738, -122.438822),
    new google.maps.LatLng(37.780201, -122.438882),
    new google.maps.LatLng(37.780400, -122.438905),
    new google.maps.LatLng(37.780501, -122.438921),
    new google.maps.LatLng(37.780892, -122.438986),
    new google.maps.LatLng(37.781446, -122.439087),
    new google.maps.LatLng(37.781985, -122.439199),
    new google.maps.LatLng(37.782239, -122.439249),
    new google.maps.LatLng(37.782286, -122.439266),
    new google.maps.LatLng(37.797847, -122.429388),
    new google.maps.LatLng(37.797874, -122.429180),
    new google.maps.LatLng(37.797885, -122.429069),
    new google.maps.LatLng(37.797887, -122.429050),
    new google.maps.LatLng(37.797933, -122.428954),
    new google.maps.LatLng(37.798242, -122.428990),
    new google.maps.LatLng(37.798617, -122.429075),
    new google.maps.LatLng(37.798719, -122.429092),
    new google.maps.LatLng(37.798944, -122.429145),
    new google.maps.LatLng(37.799320, -122.429251),
    new google.maps.LatLng(37.799590, -122.429309),
    new google.maps.LatLng(37.799677, -122.429324),
    new google.maps.LatLng(37.799966, -122.429360),
    new google.maps.LatLng(37.800288, -122.429430),
    new google.maps.LatLng(37.800443, -122.429461),
    new google.maps.LatLng(37.800465, -122.429474),
    new google.maps.LatLng(37.800644, -122.429540),
    new google.maps.LatLng(37.800948, -122.429620),
    new google.maps.LatLng(37.801242, -122.429685),
    new google.maps.LatLng(37.801375, -122.429702),
    new google.maps.LatLng(37.801400, -122.429703),
    new google.maps.LatLng(37.801453, -122.429707),
    new google.maps.LatLng(37.801473, -122.429709),
    new google.maps.LatLng(37.801532, -122.429707),
    new google.maps.LatLng(37.801852, -122.429729),
    new google.maps.LatLng(37.802173, -122.429789),
    new google.maps.LatLng(37.802459, -122.429847),
    new google.maps.LatLng(37.802554, -122.429825),
    new google.maps.LatLng(37.802647, -122.429549),
    new google.maps.LatLng(37.802693, -122.429179),
    new google.maps.LatLng(37.802729, -122.428751),
    new google.maps.LatLng(37.766104, -122.409291),
    new google.maps.LatLng(37.766103, -122.409268),
    new google.maps.LatLng(37.766138, -122.409229),
    new google.maps.LatLng(37.766183, -122.409231),
    new google.maps.LatLng(37.766153, -122.409276),
    new google.maps.LatLng(37.766005, -122.409365),
    new google.maps.LatLng(37.765897, -122.409570),
    new google.maps.LatLng(37.765767, -122.409739),
    new google.maps.LatLng(37.765693, -122.410389),
    new google.maps.LatLng(37.765615, -122.411201),
    new google.maps.LatLng(37.765533, -122.412121),
    new google.maps.LatLng(37.765467, -122.412939),
    new google.maps.LatLng(37.765444, -122.414821),
    new google.maps.LatLng(37.765444, -122.414964),
    new google.maps.LatLng(37.765318, -122.415424),
    new google.maps.LatLng(37.763961, -122.415296),
    new google.maps.LatLng(37.763115, -122.415196),
    new google.maps.LatLng(37.762967, -122.415183),
    new google.maps.LatLng(37.762278, -122.415127),
    new google.maps.LatLng(37.761675, -122.415055),
    new google.maps.LatLng(37.760932, -122.414988),
    new google.maps.LatLng(37.759337, -122.414862),
    new google.maps.LatLng(37.773187, -122.421922),
    new google.maps.LatLng(37.773043, -122.422118),
    new google.maps.LatLng(37.773007, -122.422165),
    new google.maps.LatLng(37.772979, -122.422219),
    new google.maps.LatLng(37.772865, -122.422394),
    new google.maps.LatLng(37.772779, -122.422503),
    new google.maps.LatLng(37.772676, -122.422701),
    new google.maps.LatLng(37.772606, -122.422806),
    new google.maps.LatLng(37.772566, -122.422840),
    new google.maps.LatLng(37.772508, -122.422852),
    new google.maps.LatLng(37.772387, -122.423011),
    new google.maps.LatLng(37.772099, -122.423328),
    new google.maps.LatLng(37.771704, -122.423783),
    new google.maps.LatLng(37.771481, -122.424081),
    new google.maps.LatLng(37.771400, -122.424179),
    new google.maps.LatLng(37.771352, -122.424220),
    new google.maps.LatLng(37.771248, -122.424327),
    new google.maps.LatLng(37.770904, -122.424781),
    new google.maps.LatLng(37.770520, -122.425283),
    new google.maps.LatLng(37.770337, -122.425553),
    new google.maps.LatLng(37.770128, -122.425832),
    new google.maps.LatLng(37.769756, -122.426331),
    new google.maps.LatLng(37.769300, -122.426902),
    new google.maps.LatLng(37.769132, -122.427065),
    new google.maps.LatLng(37.769092, -122.427103),
    new google.maps.LatLng(37.768979, -122.427172),
    new google.maps.LatLng(37.768595, -122.427634),
    new google.maps.LatLng(37.768372, -122.427913),
    new google.maps.LatLng(37.768337, -122.427961),
    new google.maps.LatLng(37.768244, -122.428138),
    new google.maps.LatLng(37.767942, -122.428581),
    new google.maps.LatLng(37.767482, -122.429094),
    new google.maps.LatLng(37.767031, -122.429606),
    new google.maps.LatLng(37.766732, -122.429986),
    new google.maps.LatLng(37.766680, -122.430058),
    new google.maps.LatLng(37.766633, -122.430109),
    new google.maps.LatLng(37.766580, -122.430211),
    new google.maps.LatLng(37.766367, -122.430594),
    new google.maps.LatLng(37.765910, -122.431137),
    new google.maps.LatLng(37.765353, -122.431806),
    new google.maps.LatLng(37.764962, -122.432298),
    new google.maps.LatLng(37.764868, -122.432486),
    new google.maps.LatLng(37.764518, -122.432913),
    new google.maps.LatLng(37.763435, -122.434173),
    new google.maps.LatLng(37.762847, -122.434953),
    new google.maps.LatLng(37.762291, -122.435935),
    new google.maps.LatLng(37.762224, -122.436074),
    new google.maps.LatLng(37.761957, -122.436892),
    new google.maps.LatLng(37.761652, -122.438886),
    new google.maps.LatLng(37.761284, -122.439955),
    new google.maps.LatLng(37.761210, -122.440068),
    new google.maps.LatLng(37.761064, -122.440720),
    new google.maps.LatLng(37.761040, -122.441411),
    new google.maps.LatLng(37.761048, -122.442324),
    new google.maps.LatLng(37.760851, -122.443118),
    new google.maps.LatLng(37.759977, -122.444591),
    new google.maps.LatLng(37.759913, -122.444698),
    new google.maps.LatLng(37.759623, -122.445065),
    new google.maps.LatLng(37.758902, -122.445158),
    new google.maps.LatLng(37.758428, -122.444570),
    new google.maps.LatLng(37.757687, -122.443340),
    new google.maps.LatLng(37.757583, -122.443240),
    new google.maps.LatLng(37.757019, -122.442787),
    new google.maps.LatLng(37.756603, -122.442322),
    new google.maps.LatLng(37.756380, -122.441602),
    new google.maps.LatLng(37.755790, -122.441382),
    new google.maps.LatLng(37.754493, -122.442133),
    new google.maps.LatLng(37.754361, -122.442206),
    new google.maps.LatLng(37.753719, -122.442650),
    new google.maps.LatLng(37.753096, -122.442915),
    new google.maps.LatLng(37.751617, -122.443211),
    new google.maps.LatLng(37.751496, -122.443246),
    new google.maps.LatLng(37.750733, -122.443428),
    new google.maps.LatLng(37.750126, -122.443536),
    new google.maps.LatLng(37.750103, -122.443784),
    new google.maps.LatLng(37.750390, -122.444010),
    new google.maps.LatLng(37.750448, -122.444013),
    new google.maps.LatLng(37.750536, -122.444040),
    new google.maps.LatLng(37.750493, -122.444141),
    new google.maps.LatLng(37.790859, -122.402808),
    new google.maps.LatLng(37.790864, -122.402768),
    new google.maps.LatLng(37.790995, -122.402539),
    new google.maps.LatLng(37.791148, -122.402172),
    new google.maps.LatLng(37.791385, -122.401312),
    new google.maps.LatLng(37.791405, -122.400776),
    new google.maps.LatLng(37.791288, -122.400528),
    new google.maps.LatLng(37.791113, -122.400441),
    new google.maps.LatLng(37.791027, -122.400395),
    new google.maps.LatLng(37.791094, -122.400311),
    new google.maps.LatLng(37.791211, -122.400183),
    new google.maps.LatLng(37.791060, -122.399334),
    new google.maps.LatLng(37.790538, -122.398718),
    new google.maps.LatLng(37.790095, -122.398086),
    new google.maps.LatLng(37.789644, -122.397360),
    new google.maps.LatLng(37.789254, -122.396844),
    new google.maps.LatLng(37.788855, -122.396397),
    new google.maps.LatLng(37.788483, -122.395963),
    new google.maps.LatLng(37.788015, -122.395365),
    new google.maps.LatLng(37.787558, -122.394735),
    new google.maps.LatLng(37.787472, -122.394323),
    new google.maps.LatLng(37.787630, -122.394025),
    new google.maps.LatLng(37.787767, -122.393987),
    new google.maps.LatLng(37.787486, -122.394452),
    new google.maps.LatLng(37.786977, -122.395043),
    new google.maps.LatLng(37.786583, -122.395552),
    new google.maps.LatLng(37.786540, -122.395610),
    new google.maps.LatLng(37.786516, -122.395659),
    new google.maps.LatLng(37.786378, -122.395707),
    new google.maps.LatLng(37.786044, -122.395362),
    new google.maps.LatLng(37.785598, -122.394715),
    new google.maps.LatLng(37.785321, -122.394361),
    new google.maps.LatLng(37.785207, -122.394236),
    new google.maps.LatLng(37.785751, -122.394062),
    new google.maps.LatLng(37.785996, -122.393881),
    new google.maps.LatLng(37.786092, -122.393830),
    new google.maps.LatLng(37.785998, -122.393899),
    new google.maps.LatLng(37.785114, -122.394365),
    new google.maps.LatLng(37.785022, -122.394441),
    new google.maps.LatLng(37.784823, -122.394635),
    new google.maps.LatLng(37.784719, -122.394629),
    new google.maps.LatLng(37.785069, -122.394176),
    new google.maps.LatLng(37.785500, -122.393650),
    new google.maps.LatLng(37.785770, -122.393291),
    new google.maps.LatLng(37.785839, -122.393159),
    new google.maps.LatLng(37.782651, -122.400628),
    new google.maps.LatLng(37.782616, -122.400599),
    new google.maps.LatLng(37.782702, -122.400470),
    new google.maps.LatLng(37.782915, -122.400192),
    new google.maps.LatLng(37.783137, -122.399887),
    new google.maps.LatLng(37.783414, -122.399519),
    new google.maps.LatLng(37.783629, -122.399237),
    new google.maps.LatLng(37.783688, -122.399157),
    new google.maps.LatLng(37.783716, -122.399106),
    new google.maps.LatLng(37.783798, -122.399072),
    new google.maps.LatLng(37.783997, -122.399186),
    new google.maps.LatLng(37.784271, -122.399538),
    new google.maps.LatLng(37.784577, -122.399948),
    new google.maps.LatLng(37.784828, -122.400260),
    new google.maps.LatLng(37.784999, -122.400477),
    new google.maps.LatLng(37.785113, -122.400651),
    new google.maps.LatLng(37.785155, -122.400703),
    new google.maps.LatLng(37.785192, -122.400749),
    new google.maps.LatLng(37.785278, -122.400839),
    new google.maps.LatLng(37.785387, -122.400857),
    new google.maps.LatLng(37.785478, -122.400890),
    new google.maps.LatLng(37.785526, -122.401022),
    new google.maps.LatLng(37.785598, -122.401148),
    new google.maps.LatLng(37.785631, -122.401202),
    new google.maps.LatLng(37.785660, -122.401267),
    new google.maps.LatLng(37.803986, -122.426035),
    new google.maps.LatLng(37.804102, -122.425089),
    new google.maps.LatLng(37.804211, -122.424156),
    new google.maps.LatLng(37.803861, -122.423385),
    new google.maps.LatLng(37.803151, -122.423214),
    new google.maps.LatLng(37.802439, -122.423077),
    new google.maps.LatLng(37.801740, -122.422905),
    new google.maps.LatLng(37.801069, -122.422785),
    new google.maps.LatLng(37.800345, -122.422649),
    new google.maps.LatLng(37.799633, -122.422603),
    new google.maps.LatLng(37.799750, -122.421700),
    new google.maps.LatLng(37.799885, -122.420854),
    new google.maps.LatLng(37.799209, -122.420607),
    new google.maps.LatLng(37.795656, -122.400395),
    new google.maps.LatLng(37.795203, -122.400304),
    new google.maps.LatLng(37.778738, -122.415584),
    new google.maps.LatLng(37.778812, -122.415189),
    new google.maps.LatLng(37.778824, -122.415092),
    new google.maps.LatLng(37.778833, -122.414932),
    new google.maps.LatLng(37.778834, -122.414898),
    new google.maps.LatLng(37.778740, -122.414757),
    new google.maps.LatLng(37.778501, -122.414433),
    new google.maps.LatLng(37.778182, -122.414026),
    new google.maps.LatLng(37.777851, -122.413623),
    new google.maps.LatLng(37.777486, -122.413166),
    new google.maps.LatLng(37.777109, -122.412674),
    new google.maps.LatLng(37.776743, -122.412186),
    new google.maps.LatLng(37.776440, -122.411800),
    new google.maps.LatLng(37.776295, -122.411614),
    new google.maps.LatLng(37.776158, -122.411440),
    new google.maps.LatLng(37.775806, -122.410997),
    new google.maps.LatLng(37.775422, -122.410484),
    new google.maps.LatLng(37.775126, -122.410087),
    new google.maps.LatLng(37.775012, -122.409854),
    new google.maps.LatLng(37.775164, -122.409573),
    new google.maps.LatLng(37.775498, -122.409180),
    new google.maps.LatLng(37.775868, -122.408730),
    new google.maps.LatLng(37.776256, -122.408240),
    new google.maps.LatLng(37.776519, -122.407928),
    new google.maps.LatLng(37.776539, -122.407904),
    new google.maps.LatLng(37.776595, -122.407854),
    new google.maps.LatLng(37.776853, -122.407547),
    new google.maps.LatLng(37.777234, -122.407087),
    new google.maps.LatLng(37.777644, -122.406558),
    new google.maps.LatLng(37.778066, -122.406017),
    new google.maps.LatLng(37.778468, -122.405499),
    new google.maps.LatLng(37.778866, -122.404995),
    new google.maps.LatLng(37.779295, -122.404455),
    new google.maps.LatLng(37.781043, -122.402288),
    new google.maps.LatLng(37.781399, -122.401823),
    new google.maps.LatLng(37.781727, -122.401407),
    new google.maps.LatLng(37.781853, -122.401247),
    new google.maps.LatLng(37.781894, -122.401195),
    new google.maps.LatLng(37.782076, -122.400977),
    new google.maps.LatLng(37.782338, -122.400603),
    new google.maps.LatLng(37.782666, -122.400133),
    new google.maps.LatLng(37.783048, -122.399634),
    new google.maps.LatLng(37.783450, -122.399198),
    new google.maps.LatLng(37.783791, -122.398998),
    new google.maps.LatLng(37.784177, -122.398959),
    new google.maps.LatLng(37.784388, -122.398971),
    new google.maps.LatLng(37.784404, -122.399128),
    new google.maps.LatLng(37.784586, -122.399524),
    new google.maps.LatLng(37.784835, -122.399927),
    new google.maps.LatLng(37.785116, -122.400307),
    new google.maps.LatLng(37.785282, -122.400539),
    new google.maps.LatLng(37.785346, -122.400692),
    new google.maps.LatLng(37.765769, -122.407201),
    new google.maps.LatLng(37.765790, -122.407414),
    new google.maps.LatLng(37.765802, -122.407755),
    new google.maps.LatLng(37.765791, -122.408219),
    new google.maps.LatLng(37.765763, -122.408759),
    new google.maps.LatLng(37.765726, -122.409348),
    new google.maps.LatLng(37.765716, -122.409882),
    new google.maps.LatLng(37.765708, -122.410202),
    new google.maps.LatLng(37.765705, -122.410253),
    new google.maps.LatLng(37.765707, -122.410369),
    new google.maps.LatLng(37.765692, -122.410720),
    new google.maps.LatLng(37.765699, -122.411215),
    new google.maps.LatLng(37.765687, -122.411789),
    new google.maps.LatLng(37.765666, -122.412373),
    new google.maps.LatLng(37.765598, -122.412883),
    new google.maps.LatLng(37.765543, -122.413039),
    new google.maps.LatLng(37.765532, -122.413125),
    new google.maps.LatLng(37.765500, -122.413553),
    new google.maps.LatLng(37.765448, -122.414053),
    new google.maps.LatLng(37.765388, -122.414645),
    new google.maps.LatLng(37.765323, -122.415250),
    new google.maps.LatLng(37.765303, -122.415847),
    new google.maps.LatLng(37.765251, -122.416439),
    new google.maps.LatLng(37.765204, -122.417020),
    new google.maps.LatLng(37.765172, -122.417556),
    new google.maps.LatLng(37.765164, -122.418075),
    new google.maps.LatLng(37.765153, -122.418618),
    new google.maps.LatLng(37.765136, -122.419112),
    new google.maps.LatLng(37.765129, -122.419378),
    new google.maps.LatLng(37.765119, -122.419481),
    new google.maps.LatLng(37.765100, -122.419852),
    new google.maps.LatLng(37.765083, -122.420349),
    new google.maps.LatLng(37.765045, -122.420930),
    new google.maps.LatLng(37.764992, -122.421481),
    new google.maps.LatLng(37.764980, -122.421695),
    new google.maps.LatLng(37.764993, -122.421843),
    new google.maps.LatLng(37.764986, -122.422255),
    new google.maps.LatLng(37.764975, -122.422823),
    new google.maps.LatLng(37.764939, -122.423411),
    new google.maps.LatLng(37.764902, -122.424014),
    new google.maps.LatLng(37.764853, -122.424576),
    new google.maps.LatLng(37.764826, -122.424922),
    new google.maps.LatLng(37.764796, -122.425375),
    new google.maps.LatLng(37.764782, -122.425869),
    new google.maps.LatLng(37.764768, -122.426089),
    new google.maps.LatLng(37.764766, -122.426117),
    new google.maps.LatLng(37.764723, -122.426276),
    new google.maps.LatLng(37.764681, -122.426649),
    new google.maps.LatLng(37.782012, -122.404200),
    new google.maps.LatLng(37.781574, -122.404911),
    new google.maps.LatLng(37.781055, -122.405597),
    new google.maps.LatLng(37.780479, -122.406341),
    new google.maps.LatLng(37.779996, -122.406939),
    new google.maps.LatLng(37.779459, -122.407613),
    new google.maps.LatLng(37.778953, -122.408228),
    new google.maps.LatLng(37.778409, -122.408839),
    new google.maps.LatLng(37.777842, -122.409501),
    new google.maps.LatLng(37.777334, -122.410181),
    new google.maps.LatLng(37.776809, -122.410836),
    new google.maps.LatLng(37.776240, -122.411514),
    new google.maps.LatLng(37.775725, -122.412145),
    new google.maps.LatLng(37.775190, -122.412805),
    new google.maps.LatLng(37.774672, -122.413464),
    new google.maps.LatLng(37.774084, -122.414186),
    new google.maps.LatLng(37.773533, -122.413636),
    new google.maps.LatLng(37.773021, -122.413009),
    new google.maps.LatLng(37.772501, -122.412371),
    new google.maps.LatLng(37.771964, -122.411681),
    new google.maps.LatLng(37.771479, -122.411078),
    new google.maps.LatLng(37.770992, -122.410477),
    new google.maps.LatLng(37.770467, -122.409801),
    new google.maps.LatLng(37.770090, -122.408904),
    new google.maps.LatLng(37.769657, -122.408103),
    new google.maps.LatLng(37.769132, -122.407276),
    new google.maps.LatLng(37.768564, -122.406469),
    new google.maps.LatLng(37.767980, -122.405745),
    new google.maps.LatLng(37.767380, -122.405299),
    new google.maps.LatLng(37.766604, -122.405297),
    new google.maps.LatLng(37.765838, -122.405200),
    new google.maps.LatLng(37.765139, -122.405139),
    new google.maps.LatLng(37.764457, -122.405094),
    new google.maps.LatLng(37.763716, -122.405142),
    new google.maps.LatLng(37.762932, -122.405398),
    new google.maps.LatLng(37.762126, -122.405813),
    new google.maps.LatLng(37.761344, -122.406215),
    new google.maps.LatLng(37.760556, -122.406495),
    new google.maps.LatLng(37.759732, -122.406484),
    new google.maps.LatLng(37.758910, -122.406228),
    new google.maps.LatLng(37.758182, -122.405695),
    new google.maps.LatLng(37.757676, -122.405118),
    new google.maps.LatLng(37.757039, -122.404346),
    new google.maps.LatLng(37.756335, -122.403719),
    new google.maps.LatLng(37.755503, -122.403406),
    new google.maps.LatLng(37.754665, -122.403242),
    new google.maps.LatLng(37.753837, -122.403172),
    new google.maps.LatLng(37.752986, -122.403112),
    new google.maps.LatLng(37.751266, -122.403355)
  ];
  
  function initialize() {
    var mapOptions = {
      zoom: 13,
      center: new google.maps.LatLng(37.774546, -122.433523),
      mapTypeId: google.maps.MapTypeId.SATELLITE
    };
  
    map = new google.maps.Map(document.getElementById('map-canvas'),
        mapOptions);
  
    var pointArray = new google.maps.MVCArray(taxiData);
  
    heatmap = new google.maps.visualization.HeatmapLayer({
      data: pointArray
    });
  
    heatmap.setMap(map);
  }
  
  function toggleHeatmap() {
    heatmap.setMap(heatmap.getMap() ? null : map);
  }
  
  function changeGradient() {
    var gradient = [
      'rgba(0, 255, 255, 0)',
      'rgba(0, 255, 255, 1)',
      'rgba(0, 191, 255, 1)',
      'rgba(0, 127, 255, 1)',
      'rgba(0, 63, 255, 1)',
      'rgba(0, 0, 255, 1)',
      'rgba(0, 0, 223, 1)',
      'rgba(0, 0, 191, 1)',
      'rgba(0, 0, 159, 1)',
      'rgba(0, 0, 127, 1)',
      'rgba(63, 0, 91, 1)',
      'rgba(127, 0, 63, 1)',
      'rgba(191, 0, 31, 1)',
      'rgba(255, 0, 0, 1)'
    ]
    heatmap.set('gradient', heatmap.get('gradient') ? null : gradient);
  }
  
  function changeRadius(r) {
        /* This was the original line in the example code.
           It simply toggles between default and radius=20
           when a button is clicked. */
        //heatmap.set('radius', heatmap.get('radius') ? null : 20);
  
        /* I've replaced it with this. */
        heatmap.set('radius', r);
  }
  
  function changeOpacity(o) {
        /* This was the original line in the example code.
           It simply toggles between default and opacity=0.2
           when a button is clicked. */
        //heatmap.set('opacity', heatmap.get('opacity') ? null : 0.2);
  
        /* I've replaced it with this. (works beautifully) */
        heatmap.set('opacity', o);
  }
  
  google.maps.event.addDomListener(window, 'load', initialize);
  
      &lt;/script&gt;
    &lt;/head&gt;
  
    &lt;body&gt;
      &lt;div id="panel"&gt;
        &lt;button onclick="toggleHeatmap()"&gt;Toggle Heatmap&lt;/button&gt;
        &lt;button onclick="changeGradient()"&gt;Change gradient&lt;/button&gt;
  
        &lt;!-- The next two lines are the original buttons --&gt;
        &lt;!--button onclick="changeRadius()"&gt;Change radius&lt;/button--&gt;
        &lt;!--button onclick="changeOpacity()"&gt;Change opacity&lt;/button--&gt;
  
        &lt;!-- These are the sliders I added.  Opacity works fine.  Radius does not. --&gt;
        Radius
        &lt;input type="range" id="radiusSlider" onchange="changeRadius(radiusSlider.value)"  min="1" max="40" step="1" value="12"&gt;
        Opacity
        &lt;input type="range" id="opacitySlider" onchange="changeOpacity(opacitySlider.value)"  min="0" max="1" step=".01" value=".6"&gt;
  
      &lt;/div&gt;
      &lt;div id="map-canvas"&gt;&lt;/div&gt;
    &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>