issues

I'm on xarray 0.11.3, and currently do not have the possibility to test on master. However, the commit history suggests that this issue still exists. Please accept my apologies if I missed something.

I'm following the plotting documentation at https://xarray.pydata.org/en/stable/plotting.html#maps

When plotting two maps in the same call (exactly as written in the documentation),

p = air.isel(time=[0, 1]).plot(transform=ccrs.PlateCarree(), col='time',
                               subplot_kws={'projection': ccrs.Orthographic(-80, 35)})

for ax in p.axes.flat:
    ax.coastlines()
    ax.gridlines()

everything works fine.

However, when I only want to plot one map,

p = air.isel(time=[0]).plot(transform=ccrs.PlateCarree(),
                            subplot_kws={'projection': ccrs.Orthographic(-80, 35)})

for ax in p.axes.flat:
    ax.coastlines()
    ax.gridlines()

I get the following error:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-63-2c5a2a533878> in <module>
      1 p = air.isel(time=[0]).plot(transform=ccrs.PlateCarree(),
----> 2                             subplot_kws={'projection': ccrs.Orthographic(-80, 35)})
      3 
      4 for ax in p.axes.flat:
      5     ax.coastlines()

/home/jupyterhub/conda/envs/prod/lib/python3.6/site-packages/xarray/plot/plot.py in __call__(self, **kwargs)
    550 
    551     def __call__(self, **kwargs):
--> 552         return plot(self._da, **kwargs)
    553 
    554     @functools.wraps(hist)

/home/jupyterhub/conda/envs/prod/lib/python3.6/site-packages/xarray/plot/plot.py in plot(darray, row, col, col_wrap, ax, hue, rtol, subplot_kws, **kwargs)
    185     kwargs['ax'] = ax
    186 
--> 187     return plotfunc(darray, **kwargs)
    188 
    189

/home/jupyterhub/conda/envs/prod/lib/python3.6/site-packages/xarray/plot/plot.py in newplotfunc(darray, x, y, figsize, size, aspect, ax, row, col, col_wrap, xincrease, yincrease, add_colorbar, add_labels, vmin, vmax, cmap, center, robust, extend, levels, infer_intervals, colors, subplot_kws, cbar_ax, cbar_kwargs, xscale, yscale, xticks, yticks, xlim, ylim, norm, **kwargs)
    852                              vmax=cmap_params['vmax'],
    853                              norm=cmap_params['norm'],
--> 854                              **kwargs)
    855 
    856         # Label the plot with metadata

/home/jupyterhub/conda/envs/prod/lib/python3.6/site-packages/xarray/plot/plot.py in pcolormesh(x, y, z, ax, infer_intervals, **kwargs)
   1106             y = _infer_interval_breaks(y, axis=0)
   1107 
-> 1108     primitive = ax.pcolormesh(x, y, z, **kwargs)
   1109 
   1110     # by default, pcolormesh picks "round" values for bounds

/home/jupyterhub/conda/envs/prod/lib/python3.6/site-packages/matplotlib/__init__.py in inner(ax, data, *args, **kwargs)
   1808                         "the Matplotlib list!)" % (label_namer, func.__name__),
   1809                         RuntimeWarning, stacklevel=2)
-> 1810             return func(ax, *args, **kwargs)
   1811 
   1812         inner.__doc__ = _add_data_doc(inner.__doc__,

/home/jupyterhub/conda/envs/prod/lib/python3.6/site-packages/matplotlib/axes/_axes.py in pcolormesh(self, alpha, norm, cmap, vmin, vmax, shading, antialiased, *args, **kwargs)
   6011         if (not isinstance(t, mtransforms.Transform) and
   6012             hasattr(t, '_as_mpl_transform')):
-> 6013             t = t._as_mpl_transform(self.axes)
   6014 
   6015         if t and any(t.contains_branch_seperately(self.transData)):

lib/cartopy/_crs.pyx in cartopy._crs.CRS._as_mpl_transform()

ValueError: Axes should be an instance of GeoAxes, got <class 'matplotlib.axes._subplots.AxesSubplot'>

It is not clear (from the user perspective) why one should work and the other should not.

Of course I can create the axes instance before using xarray's plot function, but it would be nicer if I wouldn't have to.

I'm using xarray 0.9.1.

When I have an array

<xarray.Dataset>
Dimensions:       (category: 1, lat: 672, lon: 720, species: 1)
Coordinates:
  * lat           (lat) float64 30.03 30.09 30.16 30.22 30.28 30.34 30.41 ...
  * category      (category) <U3 'agr'
  * species       (species) <U4 'PM10'
  * lon           (lon) float64 -29.94 -29.81 -29.69 -29.56 -29.44 -29.31 ...
Data variables:
    TNO-MACC3_EU  (lon, lat) float64 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ...

and try to write to netCDF using .to_netcdf(), I get an

IndexError: tuple index out of range

I can fix this by manually doing

d_['species'] = np.atleast_1d(d_['species'])
d_['category'] = np.atleast_1d(d_['category'])

before calling .to_netcdf().

I believe it would be good if xarray would do this check on its own and apply the coorection on-the-fly to allow saving to netCDF in this case as well.

I'd like to create an empty DataArray, i.e., one with only NA values. The docstring of DataArray says that data=None is allowed, if a dataset argument is provided. However, the docstring doesn't say anything about a dataset argument. 1. I think there's a bug in the docstring 2. I'd like to pass data=None and get a DataArray with the coords/dims set up properly (as defined by the coords and dims kwargs), but with a values array of NA.

The DataArray object would benefit from functionality similar to iris.cube.Cube.slices. Given an array

```

Out23: Coordinates: * sza (sza) float64 0.0 36.87 53.13 60.0 72.54 75.52 81.37 87.13 88.28 * vza (vza) float64 0.0 72.54 * raa (raa) float64 0.0 60.0 90.0 120.0 180.0 * wl (wl) float64 360.0 380.0 400.0 420.0 440.0 ```

it would be nice to be able to do

for sl in data.slices(["raa", "wl"]): # do magic with a DataArray of coordinates (sza, vza)

For some applications, it would be nice if it were possible to create an xarray.Dataset from a netCDF4.Dataset, so that the following code would work:

with Dataset(filename) as nc:
    ds = xr.open_dataset(nc)

It would be helpful if the documentation had an entry (for example, in the FAQ) about how to properly cite xarray for a scientific publication. I personally like the way e.g. the ipython folks are doing it, they have bibtex code to copy'n'paste (see https://ipython.org/citing.html).

This issue is related to #290, but addresses the general problem and not a specific way.

when manually setting the units attribute of a DataArray containing datetime types, the error

ValueError: Failed hard to prevent overwriting key 'units'

is raised.

It would be good to allow custom units, because currently the units seem to be days since FIRST_TIMESTEP, which leads to different units for different variables within the same Dataset.

fix link to mailing list

adds diff method to both DataArray and Dataset. closes #490

Still to be done: - tests

Possible enhancements: - allow numeric axis instead of dim for DataArray objects. The signature would then be DataArray.diff(dim=None, n=1, axis=None) and I would need to check that exactly one of dim and axis is None. I find it a bit ugly; DataArray.diff(n=1, dim=None, axis=None) would be nicer in my view. But then the signatures for DataArray.diff and Dataset.diff would be different, which is also not nice. - allow specifying the new coordinate array explicitly via a coord kwarg instead of just taking the coordinate values of the upper bounds.

What do you think?

I think it would be useful if the DataArray class had a diff method, equivalent to the pandas.DataFrame.diff method.

On xray 0.5.1, writing a Dataset with a non-1D-datetime variable to netCDF doesn't work:

``` times = xray.DataArray([[datetime.datetime.now()] * 2]) print times <xray.DataArray (dim_0: 1, dim_1: 2)> array([['2015-08-19T14:27:50.911502000+0200', '2015-08-19T14:27:50.911502000+0200']], dtype='datetime64[ns]') Coordinates: * dim_0 (dim_0) int64 0 * dim_1 (dim_1) int64 0 1

times.to_dataset().to_netcdf('test.nc') ```

raises

TypeError: Cannot convert input to Timestamp

Maybe I'm just missing something ... but shouldn't

xray.DataArray(np.asarray([1., np.nan]), coords=[[1, 2]], dims=["X"]).mean()

return 1 instead of nan? Or how is calculations with missing values supposed to work?

I'm running xray 0.3.0.