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Make singular colorbars consistent with single-value mappables. #26307

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Make singular colorbars consistent with single-value mappables. #26307

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4 changes: 4 additions & 0 deletions doc/api/next_api_changes/behavior/26307-AL.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,4 @@
Colorbars of single-value norms now map that value to the lowest color, not the midpoint color
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This behavior is consistent with e.g. how `~.Axes.imshow` handle inputs with
``vmin == vmax``.
9 changes: 7 additions & 2 deletions lib/matplotlib/colorbar.py
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Expand Up @@ -1088,8 +1088,13 @@ def _process_values(self):
# If we still aren't scaled after autoscaling, use 0, 1 as default
self.norm.vmin = 0
self.norm.vmax = 1
self.norm.vmin, self.norm.vmax = mtransforms.nonsingular(
self.norm.vmin, self.norm.vmax, expander=0.1)

# Only expand vmax if needed, to match Normalize's behavior of mapping
# everything to 0 if the norm is singular.
vmin, vmax = sorted([self.norm.vmin, self.norm.vmax])
self.norm.vmin = vmin
_, self.norm.vmax = mtransforms.nonsingular(vmin, vmax, expander=0.1)
Comment on lines +1094 to +1096
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This forces self.norm.vmin <= self.norm.vmax now I think. Would a user care if they had defined their norm reversed and we are now swapping this for them?

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The previous version (just calling nonsingular) already performed that ordering; I'm not changing that behavior (which I agree we can question), just doing it explicitly first to make it easier to just expand on the top side.


if (not isinstance(self.norm, colors.BoundaryNorm) and
(self.boundaries is None)):
b = self.norm.inverse(b)
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Binary file removed lib/matplotlib/tests/baseline_images/test_colorbar/colorbar_single_scatter.png
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17 changes: 8 additions & 9 deletions lib/matplotlib/tests/test_colorbar.py
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Expand Up @@ -265,18 +265,17 @@ def test_gridspec_make_colorbar():
plt.subplots_adjust(top=0.95, right=0.95, bottom=0.2, hspace=0.25)


@image_comparison(['colorbar_single_scatter.png'], remove_text=True,
savefig_kwarg={'dpi': 40})
def test_colorbar_single_scatter():
# Issue #2642: if a path collection has only one entry,
# the norm scaling within the colorbar must ensure a
# finite range, otherwise a zero denominator will occur in _locate.
# Issue #2642: If a path collection has only one entry, the norm scaling within the
# colorbar must ensure a finite range, otherwise a zero denominator will occur in
# _locate. Also, adding the colorbar must not change the value's normalization.
plt.figure()
x = y = [0]
z = [50]
cmap = mpl.colormaps['jet'].resampled(16)
cs = plt.scatter(x, y, z, c=z, cmap=cmap)
v = 50
cs = plt.scatter([0], [0], c=[50])
old_normed = cs.norm(v)
plt.colorbar(cs)
new_normed = cs.norm(v)
assert old_normed == new_normed


@pytest.mark.parametrize('use_gridspec', [True, False])
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