Fix overlapping points along the axes

_.isEqual considers +0 and -0 different, so sometimes if the points were
along an axis they would not be seen as in the same position, so they
would be allowed to overlap. The fix was switching to kpoint.equal.

Test Plan:
* Open an interactive graph
* For both linear function and movable points
  * Try to overlap the points along an axis
  * fail to do so

Auditors: jack