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.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: yelirekim

Maniphest Tasks: T13279

Differential Revision: https://secure.phabricator.com/D20502