scf.pl.trajectory: Arrow size linked to dpi

[rwiegan]

Hi @LouisFaure,

I've noticed that the arrows tend to become very large when a low dpi is set.

Is there a reason why the units for ax.quiver are set to "dots"?
Setting units="width" worked much better for me since it depends on the image size rather than its resolution.

I would be grateful if you could consider changing this.

Thanks a lot.

Best,
René