title: Log scale density coloring for geom_hex
body: <p>The <code>geom_hex</code> geometry in <code>ggplot2</code> colors hexagonal bins according to the number of points falling within them. This works pretty well for uniformly distributed data, but not so well if some regions are way more dense than others-- differences can get drowned out by the presence of a single very dense hexagon. </p>
  
  <p>How can I make the density color scale use a log scale or some other kind of normalizing transformation?</p>