I love this. Such a better way of showing distributions.
p.s. From my computer's dictionary:
choropleth map | ˈkôrəˌpleTH ˌmap | noun 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. ORIGIN 1930s: choropleth from Greek khōra ‘region’ + plēthos ‘multitude’.