title: what does ..level.. mean in ggplot::stat_density2d
body: <p>I've seen some examples when constructing a heatmap of having the <code>fill</code> variable set to <code>..level..</code>.</p>
  
  <p>Such as in this example: </p>
  
  <pre><code>library(MASS)
  ggplot(geyser, aes(x = duration, y = waiting)) + 
      geom_point() + 
      geom_density2d() + 
      stat_density2d(aes(fill = ..level..), geom = "polygon")
  </code></pre>
  
  <p>I suspect that the <code>..level..</code> means that the <code>fill</code> is set to the relative amount of layers present? Also could someone link me a good example of how to interpret these 2D-density plots, what does each contour represent etc.? I have searched online but couldn't find any suitable guide. </p>