In stacked area charts, group nearby points so they don't overlap

Summary: Ref T13279. We currently draw a point on the chart for each datapoint, but this leads to many overlapping circles. Instead, aggregate the raw points into display points ("events") at the end.

Test Plan: Viewed a stacked area chart with many points, saw a more palatable number of drawn dots.

Subscribers: yelirekim

Maniphest Tasks: T13279

Differential Revision: https://secure.phabricator.com/D20814