title: Where do I find information on the "..count.." and "..group.." special variables in ggplot2?
body: <p>Inspired by <a href="https://stackoverflow.com/questions/31200254/let-ggplot2-histogram-show-classwise-percentages-on-y-axis">this question</a></p>
  
  <p>Apparantly, there is some of way to calculate groupwise percentages in histograms using special variables like "..count.." and "..group.."</p>
  
  <p>Example from link:</p>
  
  <pre><code>ggplot(data, aes(carat, fill=color)) +
    geom_histogram(aes(y=c(..count..[..group..==1]/sum(..count..[..group..==1]),
                           ..count..[..group..==2]/sum(..count..[..group..==2]))*100),
                   position='dodge', binwidth=0.5) +
    ylab("Percentage") + xlab("Carat")
  </code></pre>
  
  <p>Is there any place I can read about these special variables and how they function/interact? My google searches have come up short and I have not been able to find anything through the apropos-command. </p>