Plotting when Interval coordinate is timedelta-based

[gerritholl]

Is your feature request related to a problem? Please describe.

The xarray plotting interface supports coordinates containing pandas.Interval iff those intervals contain numbers. It fails when those intervals contain pandas.Timedelta:

import numpy as np
import pandas as pd
import xarray as xr

da = xr.DataArray(
        np.arange(10),
        dims=("x",),
        coords={"x": [pd.Interval(i, i+1) for i in range(10)]})
da.plot()  # works

da = xr.DataArray(
        np.arange(10),
        dims=("x",),
        coords={"x": [pd.Interval(
            d-pd.Timestamp("2000-01-01"),
            d-pd.Timestamp("2000-01-01")+pd.Timedelta("1H"))
            for d in pd.date_range("2000-01-01", "2000-01-02", 10)]})
da.plot()  # fails

The latter fails with:

Traceback (most recent call last):
  File "mwe82.py", line 18, in <module>
    da.plot()  # fails
  File "/data/gholl/miniconda3/envs/py38/lib/python3.8/site-packages/xarray/plot/plot.py", line 446, in __call__
    return plot(self._da, **kwargs)
  File "/data/gholl/miniconda3/envs/py38/lib/python3.8/site-packages/xarray/plot/plot.py", line 200, in plot
    return plotfunc(darray, **kwargs)
  File "/data/gholl/miniconda3/envs/py38/lib/python3.8/site-packages/xarray/plot/plot.py", line 302, in line
    _ensure_plottable(xplt_val, yplt_val)
  File "/data/gholl/miniconda3/envs/py38/lib/python3.8/site-packages/xarray/plot/utils.py", line 551, in _ensure_plottable
    raise TypeError(
TypeError: Plotting requires coordinates to be numeric or dates of type np.datetime64, datetime.datetime, cftime.datetime or pd.Interval.

This error message is somewhat confusing, because the coordinates are "dates of type (...) pd.Interval", but perhaps a timedelta is not considered a date.

Describe the solution you'd like

I would like that I can use the xarray plotting interface for any pandas.Interval coordinate, including pandas.Timestamp and pandas.Timedelta.

Describe alternatives you've considered

I'll "manually" calculate the midpoints and use those as a timedelta coordinate instead.

Additional context

It seems that regular timedeltas aren't really supported either, although they don't cause an error message, they rather produce incorrect results. There's probably a related issue somewhere, but I can't find it now.

[mathause]

Quickly looking at the code this error message seems to not be entirely correct. That should probably be along the lines of

TypeError: Plotting requires coordinates to be numeric or dates of type
np.datetime64, datetime.datetime, cftime.datetime
- or pd.Interval.
+ or pd.Interval containing one of these dtypes.

The pd.Interval are used to calculate their midpoints. Then the dtype of the midpoints is checked:

xarray/xarray/plot/utils.py

Line 578 in 9c85dd5

def _ensure_plottable(*args):

But indeed matplotlib does not seem to be able to directly plot pd.Timedeltas

x = [d-pd.Timestamp("2000-01-01") for d in pd.date_range("2000-01-01", "2000-01-02", 10)]
plt.plot(x, np.arange(10))
TypeError: ...

So I am not sure we should try to be clever here.

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