matplotlib backend does not handle multigraphs / reciprocal edges

[szhorvat]

Describe the bug

Unlike the Cairo backend, the matplotlib backend does not handle multi-edges, or even reciprocal edges in directed graphs. Edges are plotted on top of each other, making it impossible to label or style them separately.

To reproduce

import igraph as ig
import matplotlib.pyplot as plt

g = ig.Graph([(0,1), (1,0), (1,2), (2,0)], directed=True)

fig, ax = plt.subplots()
ig.plot(g, target=ax)

Compare with Cairo:

ig.plot(g, bbox=(300,300))

Version information

Current main, 28647e7

[szhorvat]

@iosonofabio I do see code for curved edges in the matplotlib backend. Is this a bug or a missing feature? Or is an explicit setting needed for curving edges?

[szhorvat]

#395 states that this used to work, so it appears that this is a regression? @ntamas, if so, can we squeeze a fix into 0.10.5? This is quite important functionality.

[szhorvat]

FYI this still works correctly in the released 0.10.4 and is only broken on the latest main. This should be a blocker for the 0.10.5 release.