Returns a Bokeh glyph object and optionally creates a colorbar.

def _init_glyph(self, plot, mapping, properties):
        """
        Returns a Bokeh glyph object and optionally creates a colorbar.
        """
        ret = super(ColorbarPlot, self)._init_glyph(plot, mapping, properties)
        if self.colorbar:
            for k, v in list(self.handles.items()):
                if not k.endswith('color_mapper'):
                    continue
                self._draw_colorbar(plot, v, k[:-12])
        return ret