title: R set plot background based on date
body: <p>I have a chart of financial activity and a couple running sums.<img src="https://i.stack.imgur.com/7v2lV.png" alt="enter image description here">  Things are getting a little busy and I'm having trouble distinguishing fiscal (ends June 30th) vs calendar year.  Is there a way to set the background to different colors based on date?</p>
  
  <p>In other words could I set background to lite green where 2009-06-30 &lt; date &lt; 2010-07-01?</p>