title: ggplot2: horizontal position of stat_summary with geom_boxplot
body: <p>I want to annotate mean of each boxplot using <code>ggplot2</code>. However, I could not figure out how to horizontally center the symbols marking the means within their respective boxes (see image below).</p>
  
  <p>MWE is below for reference:</p>
  
  <pre><code>library(ggplot2)
  ggplot(data=mpg, mapping=aes(x=class, y=hwy)) + 
    geom_boxplot(aes(color = drv), outlier.shape = NA) +
    stat_summary(fun.y = mean, geom = "point", size=2, aes(shape = drv, color = drv)) + 
    theme_bw()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/IgIex.png" rel="noreferrer"><img src="https://i.stack.imgur.com/IgIex.png" alt="enter image description here"></a></p>