interactive-graph: fix js error in click-to-add-points
interactive-graph: fix js error in click-to-add-points

Summary:
We weren't passing the grab coord to Movable.grab() from
MovablePoint.grab()

Test Plan:
 * Make an unlimited-points interactive-graph
 * Click on both graphs
 * See no js error (this happens without this commit)
 * Also test grading

Auditors: charlie

iamchenxin/perseus,learningequality/perseus,daukantas/perseus,sachgits/perseus,learningequality/perseus,abe4mvp/perseus,chirilo/perseus,ariabuckles/perseus,ariabuckles/perseus,alexristich/perseus,alexristich/perseus,ariabuckles/perseus,abe4mvp/perseus,chirilo/perseus,iamchenxin/perseus,ariabuckles/perseus,daukantas/perseus,sachgits/perseus