title: normalize ggplot histogram so that first height is 1 (to show growth) in R
body: <p>I was wondering if there is a way to normalize the heights of the histograms with multiple groups so that their first heights are all = 1. For instance:</p>
  
  <pre><code>results &lt;- rep(c(1,1,2,2,2,3,1,1,1,3,4,4,2,5,7),3)
  category &lt;- rep(c("a","b","c"),15)
  data &lt;- data.frame(results,category)
  p &lt;- ggplot(data, aes(x=results, fill = category, y = ..count..))
  p + geom_histogram(position = "dodge")
  </code></pre>
  
  <p>gives a regular histogram with 3 groups.
  Also</p>
  
  <pre><code>results &lt;- rep(c(1,1,2,2,2,3,1,1,1,3,4,4,2,5,7),3)
  category &lt;- rep(c("a","b","c"),15)
  data &lt;- data.frame(results,category)
  p &lt;- ggplot(data, aes(x=results, fill = category, y = ..ncount..))
  p + geom_histogram(position = "dodge")
  </code></pre>
  
  <p>gives a the histogram where each group is normalized to have maximum height of 1.
  I want to get a histogram where each group is normalized to have first height of 1 (so I can show growth) but I don't understand if there is an appropriate alternative to ..ncount or ..count.. or if anyone can help me understand the structure of ..count.. I could maybe figure it out from there. 
  Thanks!</p>