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Patch edgecolor of a legend item does not follow look of figure #7375

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jorisvandenbossche opened this issue Nov 1, 2016 · 2 comments
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Patch edgecolor of a legend item does not follow look of figure #7375

jorisvandenbossche opened this issue Nov 1, 2016 · 2 comments
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v2.0.0

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@jorisvandenbossche
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It's not about the edgecolor of the legend box, but of the legend items (if they are a patch). In the following example, the clear border around the patches in the legend is a bit surprising to me, and contrasts the look of the figure itself.

Example code:

fig, ax = plt.subplots()
ax.hist(np.random.randn(1000), alpha=0.5, label='group 1')
ax.hist(np.random.randn(1000) + 2, alpha=0.5, label='group 2')
ax.legend()

index

As a notebook: http://nbviewer.jupyter.org/gist/jorisvandenbossche/68b2831ea2b87c76d944e93edc134d59

This issue now surfaces because the default changed to no border for patches, and as such for the histogram in this example (before 2.0, this would not be an issue with the default look). And it becomes more apparent due to the use of a lower alpha in this specific example.

Not sure if this is still the case with master, but I tested with matplotlib 2.0.0b4, on Python 3.5 / Ubuntu (installed from conda-forge)
@anntzer
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anntzer commented Nov 2, 2016

The culprit is https://github.com/matplotlib/matplotlib/blob/master/lib/matplotlib/patches.py#L295

self._edgecolor = colors.to_rgba(color, self._alpha)

i.e. the edgecolor of the legend patch inherits the alpha value set by default.

Fundamentally, this is again a consequence of the design choice of having alpha as a separate attribute, when it should really be part of the color spec (of course this doesn't prevent having helper functions to access and modify the alpha value).

As a stopgap measure, I think we can fix to_rgba (and friends) to have a force_alpha=True kwarg, and use force_alpha=False here in the meaning of "use the alpha value from the color itself if it specifies one". But I'd say that ultimately, removing internal use of a separate alpha attribute (which can be done without changing the public API) is the way to go.

@anntzer anntzer mentioned this issue Nov 8, 2016
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efiring added a commit to efiring/matplotlib that referenced this issue Nov 21, 2016
@efiring
BUG: in Patch.update_from, use private variables directly. Closes mat…
7a563e6 
…plotlib#7375.
@efiring efiring mentioned this issue Nov 21, 2016
Merged
NelleV added a commit that referenced this issue Nov 22, 2016
@NelleV
Merge pull request #7497 from efiring/patch_update_from
5d14495 
BUG: in Patch.update_from, use private variables.  Closes #7375.
@NelleV
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NelleV commented Nov 22, 2016

closed in #7497

@NelleV NelleV closed this as completed Nov 22, 2016
@QuLogic QuLogic added this to the 2.0 (style change major release) milestone Nov 22, 2016
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@NelleV @QuLogic @jorisvandenbossche @anntzer