The timeline graph needs to include MusicBrainz events; from the proposal:

"show MusicBrainz events like software releases and policy changes (potentially with many categories – some people care about every style change, and some people only want to see software releases, and everything in-between)"

These should likely be implemented as vertical lines, hoverable or clickable to show event details and relevant URLs; they should be able to be turned on and off as desired, along with other lines of the graph.