Fixed plotter allowing you to drag into negative values

Summary:
Dragging on the line plotter allowed you to go below the y-axis and set the column
values to negative, which shouldn't happen. Now, we cap the lowest value settable to 0.

Test Plan:
- Run Perseus.
- Create a plotter.
- Set it to 'Line' or 'Pic'.
- Drag below the y-axis and verify that grading works correctly still.

Reviewers: jack

Reviewed By: jack

Differential Revision: http://phabricator.khanacademy.org/D11360