Fix arrowheads leaving the line for non-squares

Summary:
When a linear function was plotted on an interactive graph that was not
a square, the arrowheads would leave the line itself (see gif 1)

This commit normalizes the x and y locations so the arrowheads show up
in the right position (see gif 2)

1: http://whocouldthat.be/withbug.gif
2: http://whocouldthat.be/fixed.gif
(they were too big for phabricator :( )

Test Plan:
* Load this into the ke submodule in perseus
* create an interactive graph with non-square ranges, and a line
* drag the line around to see the arrowheads stay on it

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6165