title: Change background color on just one region of the graph
body: <p>I would like to change the background color for just a portion of a graph. Is that possible?</p>
  
  <p>For example, using the following chart I might want to indicate that I am particularly interested in cars that have a weight between 2 and 4 tons and thus would want to highlight this region using a pink background.</p>
  
  <p>More specifically I'd like to overlay a transparent pink rectangle which stretches from 2 to 4 on the x axis and covers the entire y axis region.</p>
  
  <p>How would I code this?</p>
  
  <pre><code>p &lt;- ggplot(mtcars, aes(wt, mpg))
  p + geom_point()
  </code></pre>