markerEnd and variable aesthetics

[Fil]

When using markerEnd on a line with variable aesthetics, the marker is shown on each and every segment:

This seems wrong, and it would be better to apply markerStart only on the first segment, and markerEnd only the last segment. marker should also probably be applied more sparingly to avoid duplications between the end of a segment and start of the following segment.

There is a work-around, but it doesn't feel great:

see https://observablehq.com/@observablehq/plot-whirlpool-arrows

[mbostock]

Seems like applyGroupedMarkers should be able to do the “right thing” with respect to groups.