title: stat_density2d: removed rows containing non-finite values
body: <p>Hi~ I saw some discussion about this but there seems to be not much answer. So I plot a density/heatmap on a city map.</p>
  
  <pre><code>df = data.frame(lon=rnorm(1000, mean=-87.62, sd=0.01), 
                  lat=rnorm(1000, mean=41.88, sd=0.01))
  
  
  map = get_googlemap('chicago', zoom=14, color='bw')
  
  ggmap(map, extent='device') +
    stat_density2d(
      aes(x=lon, y=lat, fill=..level.., alpha =..level..),
      size = 2, bins = 4, 
      data = df,
      geom = "polygon"
    ) +
    scale_fill_gradient(low='green', high='red') +
    scale_alpha(guide=FALSE)
  </code></pre>
  
  <p>It plots alright but shows:</p>
  
  <pre><code>Warning message:
  Removed 96 rows containing non-finite values (stat_density2d).
  </code></pre>
  
  <p>Using real city crime data, it actually reported removing most rows. So it worries me maybe the plot doesn't reflect the real distribution.</p>
  
  <p>What does it mean? Should I worry that the plot doesn't reflect the data? Thanks!</p>