Copy the "line-chart" behavior to "line-chart-legacy" to keep "Maniphest > Reports" working

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