Allow labels to be customized for graph

Summary:
This commit adds fields for the x and y labels on a graph. Previously, they
were always "x" and "y".

Test Plan:
  * Add an interactive-graph
  * Try changing the labels in the "x-label" and "y-label" fields
  * repeat for transformer

Reviewers: jack

Reviewed By: jack

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D5883