pass projection argument to plt.subplot when faceting with cartopy transform

I have a data 3D DataArray with Time, Latitude and Longitude coordinates.

I want to plot maps of this dataset, faceted by Time. The following code

import cartopy.crs as ccrs
proj = ccrs.PlateCarree()
data.plot(transform=proj, col='Time', col_wrap=3, robust=True)

fails with

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

this is because to plot with a transform, the axes must be a GeoAxes, which is done with something like plt.subplot(111, projection=proj). The implicit subplotting done when faceting does not do that. To make the faceting works, I had to do

import cartopy.crs as ccrs
proj = ccrs.PlateCarree()
data.plot(transform=proj, col='Time', col_wrap=3, robust=True, subplot_kws={'projection':proj})

I propose that, when plot faceting is requested with a transform kw, the content of that keyword should be passed to the subplot function as a projection argument automatically by default. If a projection is provided explicitely like in the call above, use that one.

[fmaussion]

However it looks like no xarray code contains the transform keyword. However I definitely can use it in calls to plot(). I don’t understand what is going on. Can someone enlighten me?

Yes, this is what I meant with xarray not being directly “cartopy aware”. The reason is that xarray intends to be a general library, and tries to avoid defining too many keyword arguments in order to avoid confusion. However, xarray passes along any kw argument you want to the underlying mpl plot function (https://github.com/pydata/xarray/blob/master/xarray/plot/plot.py#L379). You’ll have to catch it first, either from the kwargs dict or by adding your own to the signature (the latter being recommended only if necessary, for the reason I mentioned above)

[mathause]

I think the second example shows this: http://xarray.pydata.org/en/stable/plotting.html#maps

Related: #3169