Returns a Bokeh glyph object.

def _init_glyph(self, plot, mapping, properties):
        """
        Returns a Bokeh glyph object.
        """
        properties = mpl_to_bokeh(properties)
        properties = dict(properties, **mapping)
        if 'xs' in mapping:
            renderer = plot.patches(**properties)
        else:
            renderer = plot.quad(**properties)
        if self.colorbar and 'color_mapper' in self.handles:
            self._draw_colorbar(plot, self.handles['color_mapper'])
        return renderer, renderer.glyph