title: R ggplot2: Add means as horizontal line in a boxplot
body: <p>I have created a boxplot using ggplot2:</p>
  
  <pre><code>library(ggplot2)
  
  dat &lt;- data.frame(study = c(rep('a',50),rep('b',50)), 
                    FPKM = c(rnorm(1:50),rnorm(1:50)))
  
  ggplot(dat, aes(x = study, y = FPKM)) + geom_boxplot()
  </code></pre>
  
  <p>The boxplot shows the median as a horizontal line across each box. </p>
  
  <p><a href="https://i.stack.imgur.com/n3P8I.png" rel="noreferrer"><img src="https://i.stack.imgur.com/n3P8I.png" alt="enter image description here"></a></p>
  
  <p>How do I add a dashed line to the box representing the mean of that group?</p>
  
  <p>Thanks!</p>