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This is pretty cool, the Google Music Timeline, which plots genres of music based on how many Google Play Music users have an artist or album in their music library.
Each stripe on the graph represents a genre; the thickness of the stripe tells you roughly the popularity of music released in a given year in that genre. Colors are used to visually separate genres and group sub-genres.
I am personally trying it for a while now but it is really slow and not loading.
You can give the tool a try at the Google Music Timeline page.
Forum discussion at Google+.
