title: Plotting density plots on the negative y axis with ggplot2, R
body: <p>I'm trying to plot densities on both sides of the x axis - trying to visualize reads mapping to both strands of a genomic region. </p>
  
  <p>I have two problems :</p>
  
  <ol>
  <li><p>I want to use <code>ggplot2</code> but </p></li>
  <li><p>I don't know how to extract the data from the plot object, or invert the plot that has already been made.</p></li>
  </ol>
  
  <p>Any help greatly appreciated!</p>