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plot.pcolormesh fails with shading='gouraud' #3002

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TomNicholas opened this issue Jun 6, 2019 · 5 comments
Closed

plot.pcolormesh fails with shading='gouraud' #3002

TomNicholas opened this issue Jun 6, 2019 · 5 comments

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@TomNicholas
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xarray.plot.pcolormesh() fails when you pass the matplotlib.pyplot.pcolormesh() keyword argument shading='gouraud' to it.

Code Sample, a copy-pastable example if possible

import matplotlib.pyplot as plt
import numpy as np
import xarray as xr

lon, lat = np.meshgrid(np.linspace(-20, 20, 5), np.linspace(0, 30, 4))
lon += lat/10
lat += lon/10

da = xr.DataArray(np.arange(20).reshape(4, 5), dims=['y', 'x'],
                  coords = {'lat': (('y', 'x'), lat),
                            'lon': (('y', 'x'), lon)})

da.plot.pcolormesh('lon', 'lat', shading='gouraud')
plt.show()

Problem description

This gives an error:

Traceback (most recent call last):
  File "mwe.py", line 17, in <module>
    da.plot.pcolormesh('lon', 'lat', shading='gouraud')
  File "/home/tegn500/Documents/Work/Code/xarray/xarray/plot/plot.py", line 721, in plotmethod
    return newplotfunc(**allargs)
  File "/home/tegn500/Documents/Work/Code/xarray/xarray/plot/plot.py", line 662, in newplotfunc
    **kwargs)
  File "/home/tegn500/Documents/Work/Code/xarray/xarray/plot/plot.py", line 864, in pcolormesh
    primitive = ax.pcolormesh(x, y, z, **kwargs)
  File "/home/tegn500/anaconda3/envs/py36/lib/python3.6/site-packages/matplotlib/__init__.py", line 1805, in inner
    return func(ax, *args, **kwargs)
  File "/home/tegn500/anaconda3/envs/py36/lib/python3.6/site-packages/matplotlib/axes/_axes.py", line 5971, in pcolormesh
    X, Y, C = self._pcolorargs('pcolormesh', *args, allmatch=allmatch)
  File "/home/tegn500/anaconda3/envs/py36/lib/python3.6/site-packages/matplotlib/axes/_axes.py", line 5559, in _pcolorargs
    C.shape, Nx, Ny, funcname))
TypeError: Dimensions of C (4, 5) are incompatible with X (6) and/or Y (5); see help(pcolormesh)

Expected Output

This should give almost the same image as in the documentation, just with smoother shading:

Figure_1
@dcherian
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dcherian commented Jun 6, 2019

I found this on googling: matplotlib/matplotlib#8422

@TomNicholas
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Oh thanks @dcherian !

That's confusing because this works:

plt.pcolormesh(da['lon'].values, da['lat'].values, da.values, shading='gouraud')

but it looks like matplotlib is the problem, I'll close this.

@dcherian
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dcherian commented Jun 6, 2019

This might be an infer_interval_breaks issue?

@TomNicholas
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I tried

da.plot.pcolormesh('lon', 'lat', shading='gouraud', infer_intervals=True)

but got the same error unfortunately.

@johnomotani
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I had the same problem. I found that

da.plot.pcolormesh('lon', 'lat', shading='gouraud', infer_intervals=False)

does work. Would be nice if shading='gouraud' worked by default though.

I guess the problem is that 'gouraud' needs the coordinate positions at the grid points, and I assume infer_intervals=True is creating coordinate values at the grid-cell corners (so an n x m DataArray is plotted using coordinates with lengh (n+1) and (m+1). I haven't looked into what infer_intervals actually does though. Would it be safe to just turn it off if shading='gouraud' is used?

@FilipeFcp FilipeFcp mentioned this issue Sep 27, 2024
Merged
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3 participants
@dcherian @johnomotani @TomNicholas