Triangle markers that end on points rather than center on them

[nicolaskruchten]

In order to make an "arrow chart" out of a scatter trace, we need to be able to have arrowhead/triangle markers whose extreme end is at the point, rather than markers centered on a point: right now if I want an arrow from 10 to 20, the tip of the triangle overshoots.

I propose we add four new markers arrowhead-* for this purpose.

[nicolaskruchten]

Hmm, possible complication: the end of the underlying line might peek through from behind.

[nicolaskruchten]

OK, as a compromise solution, let's come up with a "triangle to level" marker similar to this, so that the line won't show through as much unless the line thickness is disproportionately big compared to the marker size: