GradientLineCollection class has been added to support multicolored lines

[ufukty]

PR Summary

First 2 lines regular LineCollection lines. Sequential lines produced with GradientLineCollection. Code for this example is below.

Motive

Showing directional edges with arrows makes the image so cluttered, if the network has too much edges and nodes. So, the better way to differentiate which end is which end is drawing them with different colors. I recently shared a work at Reddit's r/dataisbeautiful community and people's reaction showed that this sort of visualization has some value in the eyes of audience.

Implementation

My implementation is not the perfect alternative. In summery, GradientLineCollection class is regular LineCollection class, although it creates that gradient effect by splitting each line into pieces smaller individually colored lines. Since, I already overly exceeded my free time, I can't dive in the deeper areas of API. I think that implementation will work great until someone figure out better way.

Example of possibility with addition

That image shows the subreddits with most hyperlinks. Orange for outgoing links, blue for incoming links. Image shows that incoming links are well distributed to larger set of communities compared to outgoing links.

Example usage

# Regular version
lc = mpl.collections.LineCollection(
    [[(0, 4), (1, 4)], [(0, 4.3), (1, 4.3)]],
    colors=["r", "b"],
    linewidths=10,
    alpha=0.8,
)

glc1 = GradientLineCollection(
    [[(0, 3), (1, 3)]],
    gradient={
        0.0: "red",
        1.0: "blue",
    },
    linewidths=10,
    alpha=0.8,
    pieces=50
)

glc2 = GradientLineCollection(
    [[(0, 2), (1, 2)]],
    gradient={
        0.0: (1, 0, 0, 1),
        0.4999: [1, 0, 0, 0],
        0.5: "#0000ff00",
        1.0: [0, 0, 1, 1],
    },
    linewidths=10,
    alpha=0.8,
    pieces=200
)

glc3 = GradientLineCollection(
    [[(0, 1), (1, 1)]],
    gradient={
        0.0: "red",
        0.2: "yellow",
        0.4: "green",
        0.6: "cyan",
        0.8: "blue",
        1.0: "magenta",
    },
    linewidths=10,
    pieces=200
)

ax = plt.gca()
ax.add_collection(lc)
ax.add_collection(glc1)
ax.add_collection(glc2)
ax.add_collection(glc3)

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).

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[ufukty]

related #6040

[jklymak]

This seems interesting, and I assume the line can be curved.

However, we already have an API for mapping between colors and numbers between 0 and 1. I think it would be better if your "gradient" argument were replaced by a colormap/norm pair for consistency with the rest of the library. I also think that would save you a lot of new code.

[ufukty]

@jklymak I don’t think lines can be curved. I thought there are only predefined cmaps. If they allow customization the way current solution does then gradients could be replaced

[jklymak]

Sure there is a whole tutorial on how to create colormaps. https://matplotlib.org/tutorials/colors/colormap-manipulation.html

[ufukty]

Thanks for link. Will look into it when I get a chance

[melissawm]

Hi @ufukty - are you interested in continuing this work? If so, please mark the PR as ready for review and let us know if you need help with the rebase, we have instructions for that here. Thanks!

[github-actions]

Since this Pull Request has not been updated in 60 days, it has been marked "inactive." This does not mean that it will be closed, though it may be moved to a "Draft" state. This helps maintainers prioritize their reviewing efforts. You can pick the PR back up anytime - please ping us if you need a review or guidance to move the PR forward! If you do not plan on continuing the work, please let us know so that we can either find someone to take the PR over, or close it.

[timhoffm]

As given, this is a nice application and is thus not well suited for core matplotlib. If somebody has interest in following up, I suggest to start a third-party package. If we go towards for lines inside core matplotlib, #28242 is the more general approach.