title: adding percentile lines to a density plot
body: <p>I have some data <code>dt = data.table(x=c(1:200),y=rnorm(200))</code> and I start with a density plot using <code>ggplot2</code>:</p>
  
  <pre><code>plot = ggplot(dt,aes(y)) + geom_density(aes(y=..density..))
  </code></pre>
  
  <p>Is there a way I can add percentile lines similar to <a href="http://www.psychometric-success.com/images/AA1301.gif" rel="noreferrer"><strong>this</strong></a>?</p>
  
  <p>If further I could shade the segments of the graph (created by the percentile lines) similar to <a href="http://wpcontent.answcdn.com/wikipedia/commons/thumb/8/8c/Standard_deviation_diagram.svg/325px-Standard_deviation_diagram.svg.png" rel="noreferrer"><strong>this</strong></a>, then that would be great!</p>