Revert "Revert "Dot plot now works on touch devices (and mobile app in particular)""

This reverts commit 9cef1ea2d155d79f1c90ad90d459f5f1ccd7b5d9.
Fixed plotter grading and interactions

Summary: Plotter was incorrectly grading because it wasn't correctly converting Graphie
coordinates to the number of dots present on the plot. Mouse interactions were also awkward
as you had to move the mouse to bind the onMouseUp event. Both of these things are now fixed.

Test Plan:
- Create a line plot
- Click and drag on some bars, just click on others
- Verify that grading works correctly with adjusted y-scales

Auditors: jack