title: Add sample total num at x-axis for each box at boxplot in ggplot2
body: <p>I can do it with classic <code>boxplot</code>. Here we use the built-in data: <code>PlantGrown</code> as example. </p>
  
  <pre><code>attach(PlantGrowth)    
  boxplot(weight~group,data=PlantGrowth,xaxt="n")
  PlantGrowthSum=ddply(PlantGrowth,.(group),summarise,sum=length(weight))
  
  &gt; PlantGrowthSum
     group sum
  1  ctrl  10
  2  trt1  10
  3  trt2  10
  
  axis(1,1:3,paste(PlantGrowthSum$group,"(",PlantGrowthSum$sum,")",sep=""))
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/uxjhU.png" alt="Boxplot with sum number for each box"></p>
  
  <p>Here is a question, how about <code>ggplot2</code>? </p>
  
  <pre><code>library(ggplot2)
  bp &lt;- ggplot(data=PlantGrowth, aes(x=group, y=weight, fill=group))
      + geom_boxplot()
      +theme(axis.text.x=element_blank())
      +theme(axis.text.x=1:3)
  bp
  </code></pre>
  
  <p>But it failed. Any clues about which parameter should be set up? </p>