[FEATURE] Directed graphs

[dr3y]

I have been working on making directed graphs in bokeh. Using networkx it is easy to create a DiGraph object and have directions in my arrows. But using from_networkx() there is no way to automatically draw arrows at the end of the graph edges.

My current work-around is to edit the path of graph.edges to make it look like an arrow. This has the bad consequence that arrows do not stay arrow-shaped if you resize the graph or zoom in.

Proposed solution:
have from_networkx() understand about DiGraph objects and allow different glyphs to appear at the ends of lines (maybe you want flat head arrows or something instead of normal arrows)

[IuryPiva]

Hey @dr3y thanks for submitting this feature request, sorry this took so long to get a response.

Have you looked at our ArrowHead model? how would your proposal differ from it?

[bryevdv]

@IuryPiva the issue with arrow heads is that they are annotations,not glyphs, which means they don't show up in legends and don't participate in hit testing. Also it would be reasonable to have automatic conversion from networkx as with undirected graphs. Currently a user would have to manually compute the angle for every arrowhead which is a very high burden (and in case of a dynamic layout possibly impossible at least from Python). I think the old issue is valid but we could do better now by breaking things down into smaller more focused issues

[mattpap]

This PR #11122 will effectively add support for directed graphs.

[sander76]

I am looking at the above pr, but I still don't have a clue how to use this to add arrows to my directed graph.
Is there some example I could borrow some logic from ?

[mattpap]

That PR added experimental support glyph decorations (a few arrow heads) to a handful of glyphs (Segment and Arc). Complete support for directed graphs will be added as part of NumFocus Small Development Grant that is currently starting. This should be a part of bokeh 3.3 or 3.4. For now you can see this example, to get a preview of how the API will look like.

[sander76]

Thanks very much ! I'll dig into it.