title: add a vertical 50% line to a ggplot2 point chart
body: <p>I have an ggplot2 point chart created like this</p>
  
  <pre><code>qplot(Index, Popularity ,data = data.slopeone.agg)
  </code></pre>
  
  <p>now I would like to add a vertical line at the half of all data points. In other words, where the integral is 0.5. I want add this line to see wich part of the data makes 50%.</p>
  
  <p>How can I achieve this in R? I know about <code>geom_vline</code> but don't know how to determine the positon of the vline.</p>
  
  <p>The data is structured like this:
  Each row has an ItemId the popularity of this item and an index for my chart to display the values sorted by popularity.</p>
  
  <pre><code>head(data.slopeone.agg)
      Item Popularity Index
  184  258 0.07695880     1
  29    50 0.07294129     2
  121  181 0.07162558     3
  203  286 0.07030986     4
  225  313 0.06500478     5
  65   100 0.06366796     6
  </code></pre>
  
  <p>my chart looks like this: <a href="http://img838.imageshack.us/img838/3194/popt.png" rel="nofollow">http://img838.imageshack.us/img838/3194/popt.png</a></p>