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

I love how efficiently we can plot line data for different observations using xr.DataArray.plot(hue={hue coordinate name}) over a 2D array, and I have appreciated xr.DataArray.plot.step() for plotting histogram data using interval coordinates. Today, I wanted to plot/compare several histograms over the same set of bins. I figured I could write xr.DataArray.plot.step(hue={...}), but I found out that this functionality is not implemented.

Describe the solution you'd like

I think we should have a hue kwarg for xr.DataArray.plot.step(). When specified, we would be able to plot 2D data in the same way as xr.DataArray.plot(), except that we get a set of step plots instead of a set of line plots.

Describe alternatives you've considered

• Use xr.DataArray.plot() instead. This is effective for histograms with many bins, but inaccurately represents histograms with coarse bins
• Manually call xr.DataArray.plot.hist() on each 1D subarray for each label on the hue coordinate, adding appropriate labels and legend. This is fine and my current solution, but I think it would be excellent to use the same shorthand that was developed for line plots.

Additional context

I didn't evaluate the other plotting functions implemented, but I suspect that others could appropriately consider a hue argument but do not yet support doing so.

• [x] Closes #6401
• [x] Tests added
• [x] User visible changes (including notable bug fixes) are documented in whats-new.rst

What happened?

I receive unnecessary warnings when opening Zarr datasets with empty dimensions/arrays using the chunks argument (for a non-empty dimension).

If an array has zero size (due to an empty dimension), it is saved as a single chunk regardless of Dask chunking on other dimensions (#5742). If the chunks parameter is provided for other dimensions when loading the Zarr file (based on the expected chunksizes were the array nonempty), xarray gives a warning about potentially degraded performance from splitting the single chunk.

What did you expect to happen?

I expect no warning to be raised when there is no data:

• performance degradation on an empty array should be negligible.
• we don't always know if one of the dimensions is empty until loading. But we would use the chunks parameter for dimensions with consistent chunksizes (to specify a multiple of what's on disk) -- this is thrown off when other dimensions are empty.

Minimal Complete Verifiable Example

```Python import xarray as xr import numpy as np

each a is expected to be chunked separately

ds = xr.Dataset({"x": (("a", "b"), np.empty((4, 0)))}).chunk({"a": 1})

but when we save it, it gets saved as a single chunk

ds.to_zarr("tmp.zarr")

so if we open it up with expected chunksizes (not knowing that b is empty):

ds2 = xr.open_zarr("tmp.zarr", chunks={"a": 1})

we get a warning :(

```

Relevant log output

Python {...}/miniconda3/envs/new-majiq/lib/python3.8/site-packages/xarray/core/dataset.py:410: UserWarning: Specified Dask chunks (1, 1, 1, 1) would separate on disks chunk shape 4 for dime nsion a. This could degrade performance. (chunks = {'a': (1, 1, 1, 1), 'b': (0,)}, preferred_ch unks = {'a': 4, 'b': 1}). Consider rechunking after loading instead. _check_chunks_compatibility(var, output_chunks, preferred_chunks)

Anything else we need to know?

This can be fixed by only calling _check_chunks_compatibility() whenever var is nonempty (PR forthcoming).

Environment

INSTALLED VERSIONS [3/1946]

commit: None python: 3.8.12 | packaged by conda-forge | (default, Jan 30 2022, 23:42:07) [GCC 9.4.0] python-bits: 64 OS: Linux OS-release: 5.4.72-microsoft-standard-WSL2 machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: C.UTF-8 LOCALE: ('en_US', 'UTF-8') libhdf5: 1.12.1 libnetcdf: None

xarray: 2022.3.0 pandas: 1.4.1 numpy: 1.22.2 scipy: 1.8.0 netCDF4: None pydap: None h5netcdf: None h5py: 3.6.0 Nio: None zarr: 2.11.1 cftime: 1.5.1.1 nc_time_axis: None PseudoNetCDF: None rasterio: None cfgrib: None iris: None bottleneck: 1.3.4 dask: 2022.01.0 distributed: 2022.01.0 matplotlib: 3.5.1 cartopy: None seaborn: 0.11.2 numbagg: None fsspec: 2022.01.0 cupy: None pint: None sparse: None setuptools: 59.8.0 pip: 22.0.4 conda: None pytest: 7.0.1 IPython: 8.1.1 sphinx: 4.4.0

I noticed that {DataArray,Dataset}.sortby() are implemented using np.lexsort(), which is a stable sort. Can we expect this function to remain a stable sort in the future even if the implementation is changed for some reason?

It is not explicitly stated in the docs that the sorting will be stable. If this function is meant to always be stable, I think the documentation should explicitly state this. If not, I think it would be helpful to have an optional argument to ensure that the sort is kept stable in case the implementation changes in the future.

What happened: On a device with no display available, importing xarray without setting the matplotlib backend hangs on import of matplotlib.pyplot since #5794 was merged.

What you expected to happen: I expect to be able to run import xarray without needing to mess with environment variables or import matplotlib and change the default backend.

Revert pydata/xarray#5794, which causes failure to import when used without display (issue #6062).

• [ ] Closes #6102
• [ ] Closes #6062

What happened: Following guidance on how to use regions keyword in xr.Dataset.to_zarr(), I wrote a multithreaded program that makes independent writes to each index along an axis. But, when I use more than one thread, some of these writes fail.

What you expected to happen: I expect all the writes to take place safely so long as the regions I write to do not overlap (they do not).

Minimal Complete Verifiable Example:

```python path = "tmp.zarr" NTHREADS = 4 # when 1, things work as expected import multiprocessing.dummy as mp # threads, instead of processes

import numpy as np import dask.array as da import xarray as xr

dummy values for metadata

xr.Dataset( {"x": (("a", "b"), -da.ones((10, 7), chunks=(None, 1)))}, {"apple": ("a", -da.ones(10, dtype=int, chunks=(1,)))}, ).to_zarr(path, mode="w", compute=False)

actual values to save

ds = xr.Dataset( {"x": (("a", "b"), np.random.uniform(size=(10, 7)))}, {"apple": ("a", np.arange(10))}, )

save them using NTHREADS

with mp.Pool(NTHREADS) as p: p.map( lambda idx: ds.isel(a=slice(idx, 1 + idx)).to_zarr(path, mode="r+", region=dict(a=slice(idx, 1 + idx))), range(10) ) ds_roundtrip = xr.open_zarr(path).load() # open what we just saved over multiple threads

perfect match for x on some slices of a, but when NTHREADS > 1, x has very different value or NaN on other slices of a

xr.testing.assert_allclose(ds, ds_roundtrip) # fails when NTHREADS > 1. ```

Anything else we need to know?:

• this behavior is the same if coordinate "apple" (over a) is changed to be coordinate "a" (index over dimension)
• if dummy dataset had "apple" defined using dask, I observed ds_roundtrip having all correct values of "apple" (but not "x"). But, if it was defined as a numpy array, I observed ds_roundtrip having incorrect values of "apple" (in addition to "x").

Environment:

This fixes #5741 by loading to memory all variables with zero length before saving with Dataset.to_zarr()

• [x] Closes #5741
• [x] Tests added
• [x] Passes pre-commit run --all-files
• [x] User visible changes (including notable bug fixes) are documented in whats-new.rst

What happened: I have an xr.Dataset with a dask-array-valued variable including zero-length dimension (other variables are non-empty). I tried saving it to zarr, but it fails with a zero division error.

What you expected to happen: I expect it to save without any errors.

Minimal Complete Verifiable Example: the following commands fail.

```python import numpy as np import xarray as xr

ds = xr.Dataset( {"x": (("a", "b", "c"), np.empty((75, 0, 30))), "y": (("a", "c"), np.random.normal(size=(75, 30)))}, {"a": np.arange(75), "b": [], "c": np.arange(30)}, ).chunk({})

ds.to_zarr("fails.zarr") # RAISES ZeroDivisionError ```

Anything else we need to know?: If we load all the empty arrays to numpy, it is able to save correctly. That is:

python ds["x"].load() # run on all variables that have a zero dimension ds.to_zarr("works.zarr") # successfully runs

I'll make a PR using this solution, but not sure if this is a deeper bug that should be fixed in zarr or in a nicer way.

Environment:

• [x] Closes #3748
• [x] Tests added
• [x] Passes isort -rc . && black . && mypy . && flake8
• [x] Fully documented, including whats-new.rst for all changes and api.rst for new API

MCVE Code Sample

```python import xarray as xr

create data array with named dimension and named coordinate

x = xr.DataArray([1], {"idx": [2], "y": ("idx", [3])}, ["idx"], name="x")

what's our current index? (idx, this is fine)

x.indexes

prints "idx: Int64Index([2], dtype='int64', name='idx')"

swap dim so that y is our dimension, what's index now?

x.swap_dims({"idx": "y"}).indexes

prints "y: Int64Index([3], dtype='int64', name='idx')"

``` The dimension name is appropriately swapped but the pandas index name is incorrect.

Expected Output

```python

swap dim so that y is our dimension, what's index now?

x.swap_dims({"idx": "y"}).indexes

prints "y: Int64Index([3], dtype='int64', name='y')"

```

Problem Description

This is a problem because running x.swap_dims({"idx": "y"}).to_dataframe() gives a dataframe with columns ["x", "idx"] and index "idx". This gives ambiguous names and drops the original name, while the DataArray string representation gives no indication that this might be happening.

Output of xr.show_versions()