Interesting, thanks. BUT if I have a dataset where I have a bend value per arrow element and I say // first set bend: (d) d.bend, ... // inverse direction bend: (d) -d.bend, ... I get straight lines See https://observablehq.com/d/5acb7b52ab8bf15e (and when I rotate some arrows disappear as soon as start/end [+inset] get out of view)

Did you forget to share your notebook? I suppose you get straight lines because bend is a constant option of the arrow mark, and not a variable channel. This could be developed in the future (ref. https://github.com/observablehq/plot/issues/175). In the meantime, we could see how to make a custom mark that does this, or support arrowhead markers (http://thenewcode.com/1068/Making-Arrows-in-SVG).

Another related PR is https://github.com/observablehq/plot/pull/909 (variable styles).

Yes...sorry I forgot to share: stupid me. I definitely need to study more of Plot, i,e. channels/properties/... See my notebook fir further discussion on my goal: I am super interested in discussing about custom marks...

Is it possible to get the midpoint of the arrow arc? This could be useful in order to position a text label there...

Not directly from the arrow mark, but it should be basic trigonometry—good luck ;-)

eh, eh, not for Plot.geo: I used d3.geoInterpolate but bend changes things... (I will probably define a dataset for label positioning, rather than calculating it...)