title: R ggplot2/ggmap concentric circles as points
body: <p>I am trying to plot some information that shows full population and then a subset of that population by location on a map.  I've seen data visualizations that use concentric circles or 3-d inverted cones to convey this.  I just can't figure out how to do it in <code>ggplot</code> / <code>ggmap</code></p>
  
  <p>Here's a free hand version in <code>Paint</code> that shows a rough idea of what I'm looking to do:
  <img src="https://i.stack.imgur.com/3SBpY.jpg" alt="enter image description here"></p>
  
  <p>Here's a rough piece of data for an example:</p>
  
  <pre><code>&gt; dput(df1)
  structure(list(zip = c("00210", "00653", "00952", "02571", "04211", 
  "05286", "06478", "07839", "10090", "11559"), city = c("Portsmouth", 
  "Guanica", "Sabana Seca", "Wareham", "Auburn", "Craftsbury", 
  "Oxford", "Greendell", "New York", "Lawrence"), state = c("NH", 
  "PR", "PR", "MA", "ME", "VT", "CT", "NJ", "NY", "NY"), latitude = c(43.005895, 
  17.992112, 18.429218, 41.751554, 44.197009, 44.627698, 41.428163, 
  41.12831, 40.780751, 40.61579), longitude = c(-71.013202, -66.90097, 
  -66.18014, -70.71059, -70.239485, -72.434398, -73.12729, -74.678956, 
  -73.977182, -73.73126), timezone = c(-5L, -4L, -4L, -5L, -5L, 
  -5L, -5L, -5L, -5L, -5L), dst = c(TRUE, FALSE, FALSE, TRUE, TRUE, 
  TRUE, TRUE, TRUE, TRUE, TRUE), totalPop = c(43177, 37224, 37168, 
  15492, 1614, 88802, 2587, 80043, 78580, 87461), subPop = c(42705, 
  36926, 27556, 10827, 774, 39060, 1542, 21304, 53438, 2896)), .Names = c("zip", 
  "city", "state", "latitude", "longitude", "timezone", "dst", 
  "totalPop", "subPop"), row.names = c(1L, 50L, 200L, 900L, 1500L, 
  2000L, 2500L, 3000L, 3500L, 4000L), class = "data.frame")
  </code></pre>
  
  <p>Any suggestions?</p>