title: Coloring the background of each quadrant in a scatter plot with a different color
body: <p>Suppose I produce this scatter plot:</p>
  
  <pre><code>plot(x = runif(20,-10,10), y = runif(20,-10,10), xlim = c(-10,10), ylim = c(-10,10))
  abline(h = 0, col = "black")
  abline(v = 0, col = "black")
  </code></pre>
  
  <p>So the <code>abline</code>'s divide the plane to the four Cartesian quadrants.
  I would like to color the background of each quadrant in a different color. Say blue, red, green, and yellow for quadrants 1-4, respectively.</p>
  
  <p>Any idea?</p>