Google has introduced a new way to look at music with its launch of Music Timeline. The tool allows people to see the trends in music through time. Music Timeline draws on the song collections of millions of Google Play users to create a visualization of the popularity of various artists and genres from 1950 to today, through interactive charts. Besides tracking music patterns, it can also chart the careers of individual artists with some accuracy.

According to the About page: “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.)”

The tool presents an interesting method to quantify the way people listen to music or have listened over the years. However, Music Timeline comes with some important caveats. The tool does not necessarily project the actual popularity of music in the past; it gives a picture of what popular music people now listen to, from then. It matches in some cases, but in others, it does not.

The data to render the timeline is from Google Play, which does not represent the broader and diverse category of music fans.

“What it’s reflecting is what Google Play users have in their personal libraries, which is different from what people listen to, which in turn is different from what people historically listen to,” explains Douglas Wolk, a writer and record-label owner.

Wolk references the Echonest dataset, which powers the recommendation engines of Spotify, Rdio, and other streaming music services. Echonest could be more useful because it would tell us what people are actually listening to, not just what’s in their libraries.

“That doesn’t mean that [Music Timeline is] not fun to play with, but merely that you wouldn’t want to rely on this data for a college research paper,” suggests The Atlantic. “And you might not want to use it to settle a bet in a bar, either. At least for more granular questions.”