I love this. Such a better way of showing distributions.

p.s. From my computer's dictionary:

choropleth map | ˈkôrəˌpleTH ˌmap |
  a map that uses differences in shading, coloring, or the
  placing of symbols within predefined areas to indicate the
  average values of a property or quantity in those areas.
  Compare with isopleth.

1930s: choropleth from Greek khōra ‘region’ + plēthos ‘multitude’.