R - Background colour on ggplot split across a diagonal?
<p>Using R, library(ggplot2)</p>

<p>I have a ggplot that is a perfect square.  From the top left corner, to the bottom right corner, I have a line.  </p>

<p>So say xlim(0,100), ylim(0,100) with geom_abline(intercept=100, slope=-1).</p>

<p>How do I make everything above the line have say a blue background and below a red?</p>