Make categories mathy when setting from scale

Summary:
Previously setting categories from scale would result in plaintext. This
makes them TeX-y (and rounds them).

{F13689}

Test Plan:
* Add a plotter and go to histogram
* Set categories from scale
* See the mathy goodness

Reviewers: alex, jack

Reviewed By: jack

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D7052