title: Special variables in ggplot (..count.., ..density.., etc.)
body: <p>Consider the following lines.</p>
  
  <pre><code>p &lt;- ggplot(mpg, aes(x=factor(cyl), y=..count..))
  
  p + geom_histogram()   
  p + stat_summary(fun.y=identity, geom='bar')
  </code></pre>
  
  <p>In theory, the last two should produce the same plot. In practice, <code>stat_summary</code> fails and complains that the required y aesthetic is missing.</p>
  
  <p>Why can't I use <code>..count..</code> in <code>stat_summary</code>? I can't find anywhere in the docs information about how to use these variables.</p>