Summary:
Ref T13279. Charting changes alter how the "line-chart" behavior works, but the "Burnup Chart" still relies on the old behavior.
Although I'm intending to remove "Maniphest > Reports" once Facts is a minimally sufficient replacement, copy this behavior to keep it working until we're ready to pull the trigger.
Also fix a leftover typo from D20435.
Test Plan: Viewed a legacy Maniphest burnup rate report.
Reviewers: amckinley
Reviewed By: amckinley
Maniphest Tasks: T13279
Differential Revision: https://secure.phabricator.com/D20449
