title: percentage on y lab in a faceted ggplot barchart?
body: <p>doing facets in ggplot I would often like the percentage to be used instead of counts. </p>
  
  <p>e.g.</p>
  
  <pre><code>test1 &lt;- sample(letters[1:2], 100, replace=T)
  test2 &lt;- sample(letters[3:8], 100, replace=T)
  test &lt;- data.frame(cbind(test1,test2))
  ggplot(test, aes(test2))+geom_bar()+facet_grid(~test1)
  </code></pre>
  
  <p>This is very easy but if N is different in facet A compared to facet B, it would be better I think, to compare percentages, in such a way that the each facet sums to 100%. </p>
  
  <p>how would you achieve this?</p>
  
  <p>Hope my question makes sense.</p>
  
  <p>Sincerely.</p>