Actually, this works, but is a little unelegant 
import xarray as xr
import hvplot.xarray
import panel as pn
air_ds = xr.tutorial.open_dataset('air_temperature').load()
air = air_ds.air
hvp=air.isel(time=slice(0,4))
plot, widget = hvp.hvplot(groupby='time', width=600, widget_type='scrubber', widget_location='bottom')
widget[1][0].value = len(hvp) - 1
display(pn.Row(plot, widget))
