title: How to change color of facet borders when using facet_grid
body: <p>When using <code>facet_grid</code>, <code>ggplot2</code> divides the major categories that make up the facet variables with a broader-than-usual white line.  This serves most purposes well.  Sometimes I want to more clearly show the divisions between these major categorizations and would like to shade the facet division with another color.  Is there a way to do that?  Thanks.</p>