title: Sum percentages for each facet - respect "fill"
body: <p>I am trying to create a faceted barplot, with percentages adding up to 100 for each facet. The solution to this seems to be a combination of <code>group</code> and <code>..density..</code>. How ever - it seems to me that <code>group</code>is conflicting with <code>fill</code>.</p>
  
  <p>Data:</p>
  
  <pre><code>test &lt;- data.frame(
       test1 = sample(letters[1:2], 100, replace = TRUE), 
       test2 = sample(letters[3:8], 100, replace = TRUE)
   )
  </code></pre>
  
  <p>This gets the percentages right:</p>
  
  <pre><code>ggplot(test, aes(test2)) +
       geom_bar(aes(y = ..density.., fill=test2,group=test1)) + 
       facet_grid(~test1)
  </code></pre>
  
  <p>Bus as you can see, <code>fill</code>is overwritten:
  <img src="https://i.stack.imgur.com/SWEEU.png" alt="percentage sums to 100 for each facet"></p>
  
  <p>However, the code below respects <code>fill</code> but gives me the wrong percentages (sums to 100 for the whole chart)(using ..density..):</p>
  
  <pre><code>ggplot(test, aes(test2)) +
       geom_bar(aes(y = ..count../sum(..count..), fill=test2)) + 
       facet_grid(~test1)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/kExZv.png" alt="percentage sums to 100 for the total chart"></p>
  
  <p>Related: This old question of mine: <a href="https://stackoverflow.com/questions/4725339/percentage-on-y-lab-in-a-faceted-ggplot-barchart">percentage on y lab in a faceted ggplot barchart?</a>.</p>
  
  <p>And yes - I could create additional data, but I feel this belongs in the presentation layer. Actually this feels like a bug?</p>