Currently, the y-axis bounds for the rate-of-change graph are determined once, modified only when lines are added and removed. When zooming is done, the bounds should be recalculated given the data displayed within that zoom – this way, when data patterns have changed substantially (for example, open edits: http://musicbrainz.org/statistics/timeline/main#+-c-core-entities+r-+c-edit-information+-edit.perweek+-vote.perweek+-editor.editlastweek+-editor.votelastweek+-editor.activelastweek ) it's possible to see a useful rate-of-change graph for a smaller zoom.