Xarray non dimensional kdims

[hoxbro]

Fixes #6131

import numpy as np
import holoviews as hv
import xarray as xr
hv.extension("bokeh")

da = xr.DataArray(np.arange(6).reshape(6,1), dims=["x", "stat"], coords={"x": np.arange(6), "stat": ["mean"], "x2": ('x', np.arange(6)/6)})
hv.Curve(da, kdims="x2", vdims="stat")

TODO:

• Add test
• Check if this is the best way to do it