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plt.stairs: fix unit handling for orientation="horizontal"#31666

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stiglers-eponym:fix_stairs_units
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plt.stairs: fix unit handling for orientation="horizontal"#31666
stiglers-eponym wants to merge 1 commit into
matplotlib:mainfrom
stiglers-eponym:fix_stairs_units

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@stiglers-eponym stiglers-eponym commented May 13, 2026

PR summary

plt.stairs or Axes.stairs calls self._process_unit_info as required for e.g. time dimensions on one axis. This PR introduces a distinction between vertical and horizontal orientation to fix the unit processing for orientation="horizontal".

Example

>>> import matplotlib.pyplot as plt
>>> import numpy as np
>>> times = np.arange(10).astype("M8[Y]")
>>> plt.stairs(np.random.random(9), times, orientation="horizontal")
<matplotlib.patches.StepPatch object at 0x7f5a834a3410>
>>> plt.show()
>>> # Old behavior: sets the x axis to time coordinates, but the y axis should contain time values.
>>> # New behavior: sets the y axis to time coordinates.

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story645
story645 reviewed May 13, 2026
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Comment thread lib/matplotlib/axes/_axes.py
Comment on lines +7833 to +7835
else:
edges, values, baseline = self._process_unit_info(
[("y", edges), ("x", values), ("x", baseline)], kwargs)
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@story645 story645 May 13, 2026

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Since vertical is the default, should it be the fallback position? Or will this error out if an orientation besides horizontal or vertical is provided?

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@stiglers-eponym stiglers-eponym May 13, 2026

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I just followed the convention that is used in mpatches.StepPatch (in patches.py) to which orientation is passed. The current behavior is that any value that is not "vertical" will be interpreted like "horizontal".

One can of course change this behavior, but that was not my intention in this PR.

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@story645 story645 May 14, 2026

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I hear that that's a separate discussion. I'm also wondering why stairs has to do unit processing if it's being done inside the patch creation code. Is it cause of:

https://github.com/stiglers-eponym/matplotlib/blob/6bcc38ad70066c391e327884af8bf917975cfd3f/lib/matplotlib/axes/_axes.py#L7855-L7863

And if so can that information be populated from patch.get_data()?

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