title: Displaying separate means within fill groups in ggplot boxplot
body: <p>I have a grouped boxplot using data with 3 categories. One category is set as the x-axis of the boxplots, the other is set as the fill, and the last one, as a faceting category. I want to display the means for each fill group, but using <code>stat_summary</code> only gives me the mean for the x-axis category, without separating the means for the fill:</p>
  
  <p><a href="https://i.stack.imgur.com/PFGCE.png" rel="noreferrer"><img src="https://i.stack.imgur.com/PFGCE.png" alt="facetted boxplots"></a></p>
  
  <p>Here is the current code:</p>
  
  <pre><code>demoplot&lt;-ggplot(demo,aes(x=variable,y=value))
  demoplot+geom_boxplot(aes(fill=category2),position=position_dodge(.9))+
  stat_summary(fun.y=mean, colour="black", geom="point", shape=18, size=4,) +
  facet_wrap(~category1)
  </code></pre>
  
  <p>Is there any way to display the mean for each category2 without having to manually compute and plot the points? Adjusting the position dodge doesn't really help, as it's just one computed mean. Would creating conditions within the <code>mean()</code> function be advisable?</p>
  
  <p><a href="http://bit.ly/22poeq7" rel="noreferrer">For anyone interested, here's the data:</a></p>
  
  <p>Advanced thanks for any enlightenment on this.</p>