Violinplot with groups and hue fails displaying legend

[ahjulstad]

Violinplot tries to create the legend using the first violin only. If this violin does not have both 'hue' present (with split=True), legend creation fails.

df = pd.DataFrame({'a': np.arange(10), 'b': ['f',]*9+['g']*1, 'c': ['x',]*5 + ['y',]*5})
sns.violinplot('c', 'a', hue='b', data=df, split=True)

gives

  File "c:\users\ahju\documents\kamera\seaborn\seaborn\distributions.py", line 1293, in violinplot
    plotter.plot(ax)

  File "c:\users\ahju\documents\kamera\seaborn\seaborn\distributions.py", line 982, in plot
    self.draw_violins(ax)

  File "c:\users\ahju\documents\kamera\seaborn\seaborn\distributions.py", line 764, in draw_violins
    self.add_legend_data(ax, support[0], 0,

IndexError: index 0 is out of bounds for axis 0 with size 0

[mwaskom]

Closing #435 via #466