Topic Trends
Every song the Compass reads is named for what it is actually about, drawn from a fixed set of 30 human topics. Track those topics year over year and a shape appears: the range of what popular music is willing to be about gets wider, or it gets narrower. This is that measurement.
How this is measured
Every charting song is read once and tagged with up to three topics from a fixed set of 30. Those topics roll up into 9 broader themes, so the same data groups either way, and you can read it over the trailing 12 months or across every year tagged so far.
The Stream view carves each period into bands sized by share, so every column adds up to 100% and you can watch which subjects widen or thin as a slice of what charted. Nothing is smoothed, so a band that falls to the floor really did hit zero that period. The Point view measures the spread itself: for each period it counts every (song, topic) pairing and runs Shannon entropy over the mix, then reports an effective subject count, 2H. A period spread evenly across many subjects reads high; one where a few subjects dominate reads low. A steadily falling line is the narrowing.
One caveat: the tagger only runs forward, so earlier years stay blank until the historical Billboard catalog is tagged, and the trailing view is still filling in (daily tagging began February 2026).