arrow on curve sometimes looks weird.

[alshan]

All data and plot from here: https://aeturrell.github.io/coding-for-economists/vis-common-plots.html#connected-scatter-plot

[alshan]

Not suggesting to do like here:

but, maybe )
https://graphicdesign.stackexchange.com/questions/101996/arrow-head-brush-distorted-on-a-curved-path

[IKupriyanov-HORIS]

Fixed via c82651f.

Adjusted arrow head angle calculation logic.
How it looks now: