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[Bug]: contour on 3d plot fails if x and y are 1d and different lengths #21310

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DWesl opened this issue Oct 7, 2021 · 6 comments · Fixed by #21326
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[Bug]: contour on 3d plot fails if x and y are 1d and different lengths #21310

DWesl opened this issue Oct 7, 2021 · 6 comments · Fixed by #21326
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Documentation Good first issue Open a pull request against these issues if there are no active ones! topic: mplot3d
Milestone
v3.5.0

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@DWesl
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DWesl commented Oct 7, 2021 •
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Bug summary

Calling ax.contour(x, y, z) fails if ax was created with projection="3d" and x and y are both 1d and different lengths.

Code for reproduction

import numpy as np
import matplotlib.pyplot as plt

NX = 30
NY = 20
x = np.linspace(-10, 10, NX)
y = np.linspace(-10, 10, NY)
z = np.random.randint(0, 2, [NY, NX])

fig, ax = plt.subplots(subplot_kw={"projection": "3d"})

ax.contour(x, y, z, [0.5], zdir="z")

plt.show()

Actual outcome

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "test.py", line 12, in <module>
    ax.contour(x, y, z, [0.5], zdir="z")
  File "/usr/lib/python3.8/site-packages/mpl_toolkits/mplot3d/axes3d.py", line 2076, in contour
    self.auto_scale_xyz(X, Y, Z, had_data)
  File "/usr/lib/python3.8/site-packages/mpl_toolkits/mplot3d/axes3d.py", line 650, in auto_scale_xyz
    np.column_stack([X, Y]), not had_data)
  File "<__array_function__ internals>", line 5, in column_stack
  File "/usr/lib/python3.8/site-packages/numpy/lib/shape_base.py", line 656, in column_stack
    return _nx.concatenate(arrays, 1)
  File "<__array_function__ internals>", line 5, in concatenate
ValueError: all the input array dimensions for the concatenation axis must match exactly, but along dimension 0, the array at index 0 has size 30 and the array at index 1 has size 20

If I call plt.pause(1) after the traceback, a plot shows up showing the plot with x and y limits [0, 1], which doesn't show much that is comprehensible. If I call ax.set_xlim(-10, 10) and ax.set_ylim(-10, 10), the plot shows up fine.

Expected outcome

Changing NX to 20 produces a 3d plot with a set of contours hovering around z=0.5

Operating system

Cygwin, Windows

Matplotlib Version

3.3.3, 3.4.3

Matplotlib Backend

TkAgg, matplotlib_inline.backend_inline

Python version

3.8.10; 3.9.7

Jupyter version

No response

Other libraries

matplotlib_inline==0.1.3

Installation

conda

Conda channel

conda-forge
@DWesl
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DWesl commented Oct 7, 2021

I suspect calling np.meshgrid(X, Y) before self.auto_scale_xyz(X, Y, Z, had_data) in contour would help, given that the problem seems to be auto_scale_xyz calls np.column_stack([X.reshape(-1), Y.reshape(-1)])

@dstansby
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dstansby commented Oct 8, 2021

Yes, if you add x, y = np.meshgrid(x, y) it works fine. The action here is to fix the documentation for Axes3d.contour, which currently just says:

Input data.

Compare to the documentation for Axes.contour, which has the much more helpful:

The coordinates of the values in Z.

X and Y must both be 2D with the same shape as Z (e.g. created via numpy.meshgrid), or they must both be 1-D such that len(X) == M is the number of columns in Z and len(Y) == N is the number of rows in Z.

If not given, they are assumed to be integer indices, i.e. X = range(M), Y = range(N).

@dstansby dstansby added Documentation Good first issue Open a pull request against these issues if there are no active ones! labels Oct 8, 2021
@Kislovskiy
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@dstansby @DWesl I've opened: #21321 Could you take a look?

@DWesl DWesl mentioned this issue Oct 8, 2021
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@DWesl
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DWesl commented Oct 8, 2021

#21321 looks like it fixes the terse documentation. I made #21322 to fix the crash in Axes3D.contour.

@dstansby dstansby mentioned this issue Oct 9, 2021
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6 tasks
meeseeksmachine pushed a commit to meeseeksmachine/matplotlib that referenced this issue Oct 10, 2021
@dstansby @meeseeksmachine
Backport PR matplotlib#21321: DOC Update description of ax.contour me…
b58ea96 
…thod, resolves matplotlib#21310
@meeseeksmachine meeseeksmachine mentioned this issue Oct 10, 2021
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@jklymak
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jklymak commented Oct 11, 2021

I don't think this is closed though? ax.contour(x, y, Z) works if x and y are vectors?

@jklymak jklymak reopened this Oct 11, 2021
dstansby added a commit that referenced this issue Oct 11, 2021
@dstansby
Merge pull request #21329 from meeseeksmachine/auto-backport-of-pr-21…
98865b9 
…321-on-v3.5.x

Backport PR #21321 on branch v3.5.x (DOC Update description of ax.contour method, resolves #21310)
tacaswell pushed a commit to tacaswell/matplotlib that referenced this issue Oct 12, 2021
@dstansby
Merge pull request matplotlib#21321 from Kislovskiy/fix-21310
9a9883d 
DOC Update description of ax.contour method, resolves matplotlib#21310
@QuLogic
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QuLogic commented Oct 20, 2021

This was accidentally re-closed by the same commit getting pushed to main again.

@QuLogic QuLogic reopened this Oct 20, 2021
@QuLogic QuLogic added this to the v3.5.0 milestone Oct 21, 2021
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@QuLogic @jklymak @dstansby @DWesl @Kislovskiy