title: Multiple layers in ggplot2 with different datasets
body: <p>I have a contour plot and I would like to add a geom_path with a different set of data over it.</p>
  
  <p>Right now I have the below code, but as soon as it gets to the geom_path, it overwrites the contour plot. Is there a way to prevent this from happening?</p>
  
  <pre><code>v &lt;- ggplot(pts, aes(theta_1, theta_2, z = z))
  v + stat_contour(aes(colour = ..level..),bins=50) + xlab(expression(Theta[1])) + ylab(expression(Theta[2]))
  v+geom_path(aes(x=x,y=y,z=z), data=some.mat)
  </code></pre>