title: Add a dotted vertical line on certain x-axis values using ggplot
body: <p>The axis values are -6,-4,-2,0,2,4,6 with some y values in a density plot.</p>
  <p>Is it possible to <em>add dotted vertical lines</em> on certain x-axis values (Forex: -3 and +3 )using ggplot?</p>
  <pre><code>library(ggplot2)
  df &lt;- data.frame(x = rnorm(1000, 0, 1), y = rnorm(1000,
       0, 2), z = rnorm(1000, 2, 1.5))
  df.m &lt;- melt(df)
  ggplot(df.m) + geom_freqpoly(aes(x = value,
       y = ..density.., colour = variable))
  </code></pre>