Use graph object for The Complex Plane.

Summary: In response to D2032.

Test Plan:
locally, Chrome/OS X
- open in debug mode
- put in some incorrect answers, then a correct one
- click "Problem history" and see that the history is correct

Reviewers: eater, alpert

Reviewed By: eater

CC: stephanie

Differential Revision: http://phabricator.khanacademy.org/D2034