title: ggplot2: geom_violin(), geom_text() with facet_grid() to print total number of rows in each factor excluding NA
body: <p>In the violin plot below, I want to add total number of rows used to draw each plot excluding <code>NA</code> values. </p>
  
  <p>Input:</p>
  
  <pre><code>df &lt;- cbindX(as.data.frame(rep(c(rep("trt", 4*500), rep("trt2",4*500)),2)), 
              as.data.frame(rnorm(15*500,2)), 
              as.data.frame(c(rep("A", 8*500), rep("B", 8*500))))
  colnames(df) &lt;- c("variable", "value", "mark")
  </code></pre>
  
  <p>code:</p>
  
  <pre><code>ggplot(df,aes(x=variable,y=value)) + geom_violin(trim = T) + geom_text(aes(x = variable, y = -2, label=nrow(df)),color="red")
  </code></pre>
  
  <p>Output:
  <a href="https://i.stack.imgur.com/NL7Vo.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/NL7Vo.jpg" alt="enter image description here"></a>
  Expected output:
  <a href="https://i.stack.imgur.com/BBYDN.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/BBYDN.jpg" alt="enter image description here"></a></p>