title: How to get a vertical geom_vline to an x-axis of class date?
body: <p>Even though I found Hadley's post in the google group on <code>POSIXct</code> and <code>geom_vline</code>, I could not get it done. I have a time series from and would like to draw a vertical line for years 1998, 2005 and 2010 for example. I tried with <code>ggplot</code> and <code>qplot</code> syntax, but still I either see no vertical line at all or the vertical line is drawn at the very first vertical grid and the whole series is shifted somewhat strangely to the right.</p>
  
  <pre><code>gg &lt;- ggplot(data=mydata,aes(y=somevalues,x=datefield,color=category)) +
        layer(geom="line")
  gg + geom_vline(xintercept=mydata$datefield[120],linetype=4)
  # returns just the time series plot I had before, 
  # interestingly the legend contains dotted vertical lines
  </code></pre>
  
  <p>My date field has format "1993-07-01" and is of class <code>Date</code>.</p>