http://www.factmag.com/2014/01/19/see-how-music-genres-have-evolved-over-time-with-interactive-google-visualization/
This is the moment I wish Google could use the last.fm and discogs data. Maybe it would improve their genre definitions. Sometimes I think Google should buy last.fm, but worry it would be just as neglected as it currently is by CBS.
The thing about mapping data is it's all about understanding context.