issue_comments: 604430213

Thanks @max-sixty. Contrary to my warning about not doing a PR, I couldn't help myself and dug in a bit. It turns out that string coordinates aren't the problem, it's when the coordinate isn't in sorted order. For example, @chrisroat's original example doesn't error if the coordinate is ["G", "R"] instead of ["R", "G"]. A more concrete WIP test:

```python def test_stack_groupby_unsorted_coord(): data = [[0, 1], [2, 3]] data_flat = [0, 1, 2, 3] dims = ["y", "x"] y_vals = [2, 3]

# "y" coord is in sorted order, and everything works
arr = xr.DataArray(data, dims=dims, coords={"y": y_vals})
actual1 = arr.stack(z=["y", "x"]).groupby("z").first()
midx = pd.MultiIndex.from_product([[2, 3], [0, 1]], names=dims)
expected1 = xr.DataArray(data_flat, dims=["z"], coords={"z": midx})
xr.testing.assert_equal(actual1, expected1)

# Now "y" coord is NOT in sorted order, and the bug appears
arr = xr.DataArray(data, dims=dims, coords={"y": y_vals[::-1]})
actual2 = arr.stack(z=["y", "x"]).groupby("z").first()
midx = pd.MultiIndex.from_product([[3, 2], [0, 1]], names=dims)
expected2 = xr.DataArray(data_flat, dims=["z"], coords={"z": midx})
xr.testing.assert_equal(actual2, expected2)

test_stack_groupby_str_coords() yieldspython

AssertionError Traceback (most recent call last)

[...]

AssertionError: Left and right DataArray objects are not equal

Differing values: L array([2, 3, 0, 1]) R array([0, 1, 2, 3]) Differing coordinates: L * z (z) MultiIndex - z_leve...(z) int64 2 2 3 3 - z_leve...(z) int64 0 1 0 1 R * z (z) MultiIndex - y (z) int64 3 3 2 2 - x (z) int64 0 1 0 1 ```

I'll return to this tomorrow, in the meantime if this triggers any thoughts about the best path forward, that would be much appreciated!