da.plot.pcolormesh fails when there is a datetime coordinate

[gerritholl]

da.plot.pcolormesh, where da is a DataArray, fails with TypeError: invalid type promotion when one of the coordinates is a datetime array:

$ cat mwe.py
#!/usr/bin/env python3.6

import xarray
import numpy

da = xarray.DataArray(
    numpy.arange(3*4).reshape(3,4),
    dims=("x", "y"),
    coords={"x": [1,2,3],
            "y": [numpy.datetime64(f"2000-01-{x:02d}")
                  for x in range(1, 5)]})

da.plot.pcolormesh()
$ ./mwe.py
Traceback (most recent call last):
  File "./mwe.py", line 13, in <module>
    da.plot.pcolormesh()
  File "/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/xarray/plot/plot.py", line 547, in plotmethod
    return newplotfunc(**allargs)
  File "/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/xarray/plot/plot.py", line 500, in newplotfunc
    **kwargs)
  File "/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/xarray/plot/plot.py", line 667, in pcolormesh
    primitive = ax.pcolormesh(x, y, z, **kwargs)
  File "/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/matplotlib/__init__.py", line 1710, in inner
    return func(ax, *args, **kwargs)
  File "/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/matplotlib/axes/_axes.py", line 5636, in pcolormesh
    coords = np.column_stack((X, Y)).astype(float, copy=False)
  File "/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/numpy/lib/shape_base.py", line 353, in column_stack
    return _nx.concatenate(arrays, 1)
TypeError: invalid type promotion

[gerritholl]

I'm quite sure it worked in the past, but trying old versions of xarray yields the same error, so either my memory is wrong, or this started failing due to changes in dependencies.

[gerritholl]

Never mind, this is in matplotlib. See matplotlib/matplotlib#9577.

[gerritholl]

Actually, it isn't in matplotlib really. It's xarrays responsibility after all. To plot with pcolormesh, one needs to convert the date axis using datenum, see https://stackoverflow.com/a/27918586/974555 . When plotting with xarray, that is out of control of the user, so there must be some step within xarray to prepare this. What I still don't know is why my code (not this MWE, but my actual code) worked several months ago but not now.

[gerritholl]

The problem is triggered by a recent change in pandas. I'm currently bisecting pandas to see where it is but it's a little slow due to the compilation at every step.

[gerritholl]

This happens after: pandas-dev/pandas@2310faa

[TomAugspurger]

Pandas used to register a matplotlib converter for datetimes on import. I’ll take a closer look in a bit.

[gerritholl]

@TomAugspurger Is it this one?

[TomAugspurger]

Yep, that was the change.

The fix is to explicitly register the converters before plotting:

from pandas.tseries import converter
converter.register()