State that the data cannot be symmetric if it's empty

[maximlt]

A RuntimeWarning was emitted by numpy with the following snippet. It was trying to calculate np.nanquantile on an empty series in this case.

import pandas as pd
import hvplot.pandas

df = pd.DataFrame({'x': [], 'y': [], 'color': []})
df.hvplot(kind='scatter', x='x', y='y', color='color')

The only thing I'm unsure of is whether the data variable always has a size parameter. In other words, what type of object could data possibly be in this function?

[coveralls]

Coverage decreased (-0.01%) to 72.427% when pulling d50a52c on handle_empty_color_column into 17ce0cc on master.