Wrap "<min, max>" chart domain pairs in an "Interval" class
Summary: Ref T13279. Slightly simplify domain handling by putting all the "[x, y]" stuff in an Interval class. I'm planning to do something similar for ranges next, so this should make that easierr.
Test Plan: Viewed chart, saw same chart as before.
Reviewed By: amckinley
Maniphest Tasks: T13279
Differential Revision: https://secure.phabricator.com/D20502