title: `ggplot2`: label values of barplot that uses `fun.y="mean"` of `stat_summary`
body: <p>If I use <code>ggplot2</code>'s <code>stat_summary()</code> to make a barplot of the average number of miles per gallon for 3-, 4-, and 5-geared cars, for example, how can I label each of the bars with the average value for mpg?</p>
  
  <pre><code>library(ggplot2)
  CarPlot &lt;- ggplot() +
    stat_summary(data= mtcars,
                 aes(x = factor(gear),
                     y = mpg,
                     fill = factor(gear)
                     ),
                 fun.y="mean",
                 geom="bar"
                 )
  CarPlot
  </code></pre>
  
  <p>I know that you can normally use <code>geom_text()</code>, but I'm having trouble figuring out what to do in order to get the average value from <code>stat_summary()</code>.</p>