The horizontal axis on graphs is currently time. But points aren't randomly sampled in time -- you get a point on the graph for every changeset. As it is, this gives unfair weight to periods with few checkins. I realize that some people might want to answer "how has Firefox's performance changed over time?", but developers want to know "how has Firefox's performance changed as we've changed Firefox?". Perhaps this should be an option on the graph.
You know, this would be really tricky to do right when you have multiple data series, and there are much more important things to fix. Closing the bug.