Fix graphie lines when x and y range are different.

Summary: Also, Show "-1" on graph.

Test Plan:
I've verified this works in Perseus's interactive graph. I
tried multiple x and y scales, and the lines work (both normal and extended).
Since this is touching a lot of graphs, I'm going to need to check some
other exercises. Any suggestions on exercises to look at?

Reviewers: alpert, emily

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D3106