Allow passing figure handle to FacetGrid

Is your feature request related to a problem?

Sometimes i need to combine xarray Facet grids with other ax plots. It would be amazing if I could pass a created figure to the plot function. Event better a subfigure so that the possibilities are infinite!

Describe the solution you'd like

for example:

  da = xr.tutorial.open_dataset("air_temperature")['air']
  f = plt.figure()
  (
      da
      [{'time':[1,2,4]}]
      .plot.contourf(col='time',fig = f)
  )

Describe alternatives you've considered

an alternative is to manually to all plots in a created figure, but this becomes cumbersome.
I quickly checked the source code, and it does not seem very difficult to implement. mostly a modification to the
get_axis function so that it accepts an already created figure. I managed to quickly make it work in seaborn (see image below)

Additional context

No response

Start by reading the plotting source around get_axis, which the issue identifies as the likely entry point for accepting an existing figure. Check how FacetGrid contourf plotting currently creates axes, then define completion as allowing a created figure—and potentially a subfigure—to be passed while still supporting combination with other plots.