issues: 474463902

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.