How to use hv.dim().norm() in hvplot?

HoloViews
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1 
import xarray as xr
import hvplot.xarray
ds = xr.tutorial.open_dataset('air_temperature').mean(['lat', 'lon'])
ds.hvplot.scatter('time', color=hv.dim('air').norm())

ValueError: Cannot call method on dim('air').norm() expression. Only methods accessed via namspaces, e.g. dim(...).df or dim(...).xr), can be called.

Or more useful:

import numpy as np
import xarray as xr
import hvplot.xarray
ds = xr.tutorial.open_dataset('air_temperature').mean(['lat', 'lon'])
ds.hvplot.scatter('time', color=hv.dim('air').bin([0, 273.15, np.inf], labels=['freezing', 'above_freezing']))
2

Sort of figured it out, but how do I add a legend now?

import numpy as np
import xarray as xr
import hvplot.xarray
ds = xr.tutorial.open_dataset('air_temperature').mean(['lat', 'lon'])
ds.hvplot.scatter('time').opts(color=hv.dim('air').bin([0, 273.15, np.inf], labels=['blue', 'red']))