title: Joining means on a boxplot with a line (ggplot2)
body: <p>I have a boxplot showing multiple boxes. I want to connect the mean for each box together with a line. The boxplot does not display the mean  by default, instead the middle line only indicates the median. I tried </p>
  
  <pre><code>ggplot(data, aes(x=xData, y=yData, group=g)) 
      + geom_boxplot() 
      + stat_summary(fun.y=mean, geom="line")
  </code></pre>
  
  <p>This does not work.</p>
  
  <p>Interestingly enough, doing </p>
  
  <pre><code>stat_summary(fun.y=mean, geom="point") 
  </code></pre>
  
  <p>draws the median point in each box. Why would "line" not work?</p>
  
  <p>Something like this but using ggplot2, <a href="http://www.aliquote.org/articles/tech/RMB/c4_sols/plot45.png" rel="noreferrer">http://www.aliquote.org/articles/tech/RMB/c4_sols/plot45.png</a></p>