title: Add text below axis labels with ggplot
body: <p>I have a <code>geom_boxplot</code> to which I would like to a second set of labels to each of the axis labels below the axis. In normal plots barplots (with <code>plot</code>), I would use the arguments:</p>
  
  <pre><code>par(xpd=T)
  text(c("a", "b", "c", "d"), x=c(0.7, 1.9, 3.1, 4.3), 
          y=0, pos=1, offset=0.13)
  </code></pre>
  
  <p>Here's a sample graph using the <code>mgp</code> data showing the labels that I could want to add below the axis labels (but it is currently within the graph):</p>
  
  <pre><code>p &lt;- ggplot(mpg, aes(class, hwy))
  p + geom_boxplot()+
  annotate("text", x = c(1:7), y=-10, label = paste(1:7*10, "mpg"), cex=3)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/qX94C.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/qX94C.png" alt="enter image description here"></a></p>