Support drawing multiple functions on the same chart

Summary: Depends on D20438. Ref T13279. Widgets produced vs widgets sold, etc.

Test Plan: {F6381609}

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: yelirekim

Maniphest Tasks: T13279

Differential Revision: https://secure.phabricator.com/D20439