Returns a Bokeh glyph object.

def _init_glyph(self, plot, mapping, properties, key):
        """
        Returns a Bokeh glyph object.
        """
        properties = mpl_to_bokeh(properties)
        plot_method = '_'.join(key.split('_')[:-1])
        renderer = getattr(plot, plot_method)(**dict(properties, **mapping))
        return renderer, renderer.glyph