The 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. (For example, the "jazz" stripe is thick in the 1950s since many users' libraries contain jazz albums released in the '50s.) Click on the stripes to zoom into more specialized genres.
Explore the timelime here