facet grid axis labels are None

[jhamman]

The dim names on this plot are not showing up (e.g. None is not right, it should be x and y):

data = (np.random.random(size=(20, 25, 12)) + np.linspace(-3, 3, 12)) # range is ~ -3 to 4
da = xray.DataArray(data, dims=['x', 'y', 'time'], name='data')
fg = da.plot.pcolormesh(col='time', col_wrap=4)

[shoyer]

This comes up in facetted plots when you don't supply x and y labels explicitly. We should be able to infer these automatically from the data array dimensions, though.

[jhamman]

I'll tag the fix for this onto #608. It should be simple and it will allow me to rework the logic in _infer_xy_labels().

[clarkfitzg]

If the dimensions are A, B, C, and the call is da.plot.pcolormesh(col='B') then it seemed a little ambiguous to assign x='A', y='C'. So another option would be to raise an exception here.

[jhamman]

closed via #608.