Improve plot aspect handling when using cartopy

This applies to single plots and FacetGrids.

The current plotting behavior when using a projection that changes the plot aspect is as follows:

from xray.tutorial import load_dataset

ds = load_dataset('air_temperature')

ax = plt.subplot(projection=ccrs.LambertConformal())
ds.air.isel(time=0).plot(transform=ccrs.PlateCarree())
ax.coastlines()
ax.gridlines()

fg = ds.air.isel(time=slice(0, 9)).plot(col='time', col_wrap=3, transform=ccrs.PlateCarree(),
                                   subplot_kws=dict(projection=ccrs.LambertConformal()))
for ax in fg.axes.flat:
    ax.coastlines()
    ax.gridlines()

There are two problems here, I think both are related to the aspect of the subplot:

1. In the single case, the subplot aspect is correct but the colorbar is not scaled appropriately
2. In the FacetGrid case, the subplot aspects are not correct but the colorbar is.

[dcherian]

Found a solution for 1. This function works really well even when resizing a figure on matplotlib 3.0.2:

def make_colorbar(ax, mappable, **kwargs):
    from mpl_toolkits.axes_grid1 import make_axes_locatable
    import matplotlib as mpl

    divider = make_axes_locatable(ax)
    orientation = kwargs.pop('orientation', 'vertical')
    if orientation == 'vertical':
        loc = 'right'
    elif orientation == 'horizontal':
        loc = 'bottom'
        
    cax = divider.append_axes(loc, '5%', pad='3%', axes_class=mpl.pyplot.Axes)
    ax.get_figure().colorbar(mappable, cax=cax, orientation=orientation)

import cartopy.crs as ccrs
import xarray as xr

ds = xr.tutorial.open_dataset('air_temperature').load()

ax = plt.subplot(projection=ccrs.LambertConformal())
mappable = ds.air.isel(time=0).plot(transform=ccrs.PlateCarree(), add_colorbar=False)
ax.coastlines()
ax.gridlines()

make_colorbar(ax, mappable, orientation='vertical')

ds = xr.tutorial.open_dataset('air_temperature').load()

ax = plt.subplot(projection=ccrs.LambertConformal())
mappable = ds.air.isel(time=0).plot(transform=ccrs.PlateCarree(), add_colorbar=False)
ax.coastlines()
ax.gridlines()

make_colorbar(ax, mappable, orientation='horizontal')

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