Plotting mixed graphs

Hi!

I am fairly new to Julia but I am amazed and I started using GraphPlot. I have an idea for improvement and I would also really like to contribute with this via a PR.

However, since I am quite new to github also, I want to discuss my idea with you before going into action (and start learning how to use github properly regarding PRs meanwhile).

Mixed graphs:
It could be nice if GraphPlot could do mixed graphs too.

I think a simple and fine implementation could be via an individual mapping of the edges. In a lot of plotting cases, you have already done your mapping.

The origin of this issue, e.g., was that I wanted to plot the optimization of a transport model. I had all the flows in the graph mapped to an edge color corresponding to a flow/capacity-ratio:

Here, it could just be nice to write something like:
gplot(g,arrowlengthfrac=[abs(flow[i]) > 0 ? 0.1 : 0 for i in 1:num_of_edges])

And thereby avoiding the arrows of all the grey edges, which just disturbs the image.

I have looked into plot.jl and lines.jl, and it seems doable without too much effort and without mess up the control flow of the code.

Perspectives:
I have the impression that GraphPlot is built around LightGraphs and MixedGraph() is not yet a method. But preparing for a graph tool, in the case of a future MixedGraph() method, could be nice either way.

What do you think?
Do you find any immediate issues or should I try to start making it work?

All the best,
Benjamin

Start with src/plot.jl and src/lines.jl, especially the gplot entry point and existing edge-mapping options. Review how arrow rendering is controlled, then determine how per-edge arrow lengths could support the proposed mixed-graph visualization without disrupting current behavior. Done should include a documented approach and tests or examples covering edges with and without arrows.