title: Add a vertical line with ggplot when x-axis is a factor
body: <p>The following code use to work pre-version .9 of ggplot2. Is this not possible anymore?</p>
  <pre><code>df = data.frame(x = letters[1:26], y=abs(rnorm(26)))
  ggplot(df, aes(x=x, y=y)) + geom_bar() + geom_vline(xintercept='m')
  </code></pre>
  <p>I get the following error:</p>
  <blockquote>
  <p>Error in get(as.character(FUN), mode = &quot;function&quot;, envir = envir) :
  object 'm' of mode 'function' was not found</p>
  </blockquote>