title: How to specify gradient parameter to folium heatmap?
body: <p>I'm making a heatmap for NYC apartment price using folium. I'm trying to use my own color gradient. When I specify the gradient argument in heatmap function, nothing shows on my map. Does anyone know how to generate our own color gradient and ideally a gradient bar on the map?  Thanks a lot.</p>
  
  <p>Here is my code: 
  The first two columns of data are locations. The third column is price.</p>
  
  <pre><code>data =[[    40.7726,    -73.9568,   1900.    ],
         [    40.7785,    -73.9556,   3200.    ],
         [    40.7216,    -73.9809,   5800.    ],
         [    40.7384,    -73.9848,   2900.    ],
         [    40.7678,    -73.9915,   3312.    ],
         [    40.7659,    -73.9574,   2600.    ],
         [    40.7092,    -74.0137,   4299.    ],
         [    40.7384,    -73.982 ,   5750.    ],
         [    40.7312,    -73.9896,   3595.    ]]
  
  from folium.plugins import HeatMap
  hmap = folium.Map(location=[40.75, -73.97], tiles='stamentoner',control_scale = True, zoom_start=13)
  hmap.add_child(HeatMap(data, radius = 10, gradient={1000: 'blue', 3000: 'lime', 5000: 'red'}))
  hmap
  </code></pre>