Returns a Bokeh glyph object.

def _init_glyph(self, plot, mapping, properties, key):
        """
        Returns a Bokeh glyph object.
        """
        properties.pop('legend', None)
        if key == 'arrow':
            properties.pop('source')
            arrow_end = mapping.pop('arrow_end')
            arrow_start = mapping.pop('arrow_start')
            start = arrow_start(**properties) if arrow_start else None
            end = arrow_end(**properties) if arrow_end else None
            renderer = Arrow(start=start, end=end, **dict(**mapping))
            glyph = renderer
        else:
            properties = {p if p == 'source' else 'text_'+p: v
                          for p, v in properties.items()}
            renderer, glyph = super(ArrowPlot, self)._init_glyph(
                plot, mapping, properties, 'text_1')
        plot.renderers.append(renderer)
        return renderer, glyph