Adding quartile breaks Violin plot

[Safrone]

There is a case when rendering a Violin plot with 'quartiles' or 'stick' for the inner option will cause it to raise an exception.

Example:

import pandas as pd
import holoviews as hv

data_dict = {
    'time_of_day': {0: 16, 1: 17, 2: 18, 3: 19, 4: 20, 5: 21, 6: 22, 7: 23, 8: 0, 9: 1, 10: 2, 11: 3, 12: 4, 13: 5, 14: 6, 15: 7, 16: 8, 17: 9, 18: 10, 19: 11, 20: 12, 21: 13, 22: 14, 23: 15}, 
    'level': {0: 1, 1: 1, 2: 1, 3: 1, 4: 1, 5: 1, 6: 1, 7: 0, 8: 0, 9: 0, 10: 0, 11: 1, 12: 1, 13: 1, 14: 1, 15: 1, 16: 1, 17: 1, 18: 1, 19: 1, 20: 1, 21: 1, 22: 1, 23: 1}
}
df = pd.DataFrame(data_dict)

# Renders OK
hv.renderer('bokeh').static_html(hv.Violin(data=df, kdims='time_of_day', vdims='level'))

# Raises an exception
hv.renderer('bokeh').static_html(hv.Violin(data=d, kdims='time_of_day', vdims='level').opts(inner='quartiles'))

Exception:

Traceback (most recent call last):
  File "/venv/lib/python3.5/site-packages/IPython/core/interactiveshell.py", line 3265, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-32-77bf6a4625f1>", line 1, in <module>
    hv.renderer('bokeh').static_html(hv.Violin(data=df, kdims='time_of_day', vdims='level').opts(inner='quartiles'))
  File "/venv/lib/python3.5/site-packages/holoviews/plotting/renderer.py", line 361, in static_html
    html = self.html(obj, fmt)
  File "/venv/lib/python3.5/site-packages/holoviews/plotting/renderer.py", line 284, in html
    plot, fmt =  self._validate(obj, fmt)
  File "/venv/lib/python3.5/site-packages/holoviews/plotting/renderer.py", line 220, in _validate
    plot = self.get_plot(obj, renderer=self, **kwargs)
  File "/venv/lib/python3.5/site-packages/holoviews/plotting/bokeh/renderer.py", line 134, in get_plot
    plot = super(BokehRenderer, self_or_cls).get_plot(obj, renderer, **kwargs)
  File "/venv/lib/python3.5/site-packages/holoviews/plotting/renderer.py", line 207, in get_plot
    plot.update(init_key)
  File "/venv/lib/python3.5/site-packages/holoviews/plotting/plot.py", line 595, in update
    return self.initialize_plot()
  File "/venv/lib/python3.5/site-packages/holoviews/plotting/bokeh/element.py", line 1049, in initialize_plot
    self._init_glyphs(plot, element, ranges, source)
  File "/venv/lib/python3.5/site-packages/holoviews/plotting/bokeh/element.py", line 1192, in _init_glyphs
    data, mapping, style = self.get_data(element, ranges, style)
  File "/venv/lib/python3.5/site-packages/holoviews/plotting/bokeh/stats.py", line 411, in get_data
    kde, segs, bars, scatter = self._kde_data(g, key, **kwargs)
  File "/venv/lib/python3.5/site-packages/holoviews/plotting/bokeh/stats.py", line 347, in _kde_data
    sidx = np.argmin(np.abs(xs-stat))
  File "/venv/lib/python3.5/site-packages/numpy/core/fromnumeric.py", line 1019, in argmin
    return _wrapfunc(a, 'argmin', axis=axis, out=out)
  File "/venv/lib/python3.5/site-packages/numpy/core/fromnumeric.py", line 57, in _wrapfunc
    return getattr(obj, method)(*args, **kwds)
ValueError: attempt to get argmin of an empty sequence

This also happens for inner='stick' but not for inner='box'

[Safrone]

Still an issue for 'box'