spline size not supported

[ThierryBerger]

• When running dot -Tsvg myfile.dot (from Failed to render graph #38), I noticed a warning, it's not critical, but I just wanted to let you know. (it might be interesting to add a CI about that, and/or to upstream petgraph crate).

Warning: set_marker_node_Set(18) -> node_System(30): spline size > 1 not supported