Google found a great way to use the data from the people who uploaded songs to Google Play Music. Google Music Timeline "shows genres of music waxing and waning, based on how many Google Play Music users have an artist or album in their music library, and other data (such as album release dates). 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."
The timeline starts in 1950, data is is normalized by the total number of albums from a certain year and there's no classical music.
Mouse over a genre to find popular albums, search for an album or artist.