Activating the context menu on plots is difficult.

[tshead2]

It's hard to hit the line. We should probably be highlighting / selecting whatever is closest to the cursor.

[tshead2]

Broadly speaking, there are a couple of options:

• Add invisible geometry that can receive mouseover events.
  • Pro: just works for arbitrary complexity shapes, Bezier paths, etc.
  • Con: where there's overlap, the order in which a region is chosen is fixed.
    • In theory, getIntersectionList() could be used, but it's missing on Firefox and buggy elsewhere.
• Add spatial indexing such as kd trees.
  • Pro: Can be used to search for nearest neighbors, no matter the scene complexity.
  • Con: search for shapes other than points is trickier - at a minimum we would need nearest line segment for plots and spines, plus nearest rect for bars, plus fills.
    • Looks like maybe a quad tree is what we want.