title: Add second x-axis in ggplot2
body: <p>In the "graphics" package one can add a second x-axis (indicating the percentiles of the distribution) to a histogram as follows:</p>
  
  <pre><code>x  &lt;- rnorm(1000)
  hist(x, main="", xlab="Bias")
  perc  &lt;- quantile(x, seq(from=.00, to=1, by=.1))
  axis(1,at=perc,labels=c("0","10%","20%","30%","40%","50%","60%","70%","80%","90%","100%"),cex=0.5, pos= -90)
  </code></pre>
  
  <p>That looks awkward, of course. So how can I modify the following ggplot2 code to add a second x-axis, shwing the percentiles, while the first x-axis should indicate the raw values?:</p>
  
  <pre><code>library(ggplot2)
  theme_classic(base_size = 12, base_family = "")
  x  &lt;- rnorm(1000)
  qplot(x, main="", xlab="Bias")
  perc  &lt;- quantile(x, seq(from=.00, to=1, by=.1))
  </code></pre>
  
  <p>Any help? Many thanks in advance!</p>