Returns a Bokeh glyph object.

def _init_glyph(self, plot, mapping, properties):
        """
        Returns a Bokeh glyph object.
        """
        ret = super(SideHistogramPlot, self)._init_glyph(plot, mapping, properties)
        if not 'field' in mapping.get('fill_color', {}):
            return ret
        dim = mapping['fill_color']['field']
        sources = self.adjoined.traverse(lambda x: (x.handles.get('color_dim'),
                                                     x.handles.get('source')))
        sources = [src for cdim, src in sources if cdim == dim]
        tools = [t for t in self.handles['plot'].tools
                 if isinstance(t, BoxSelectTool)]
        if not tools or not sources:
            return
        box_select, main_source = tools[0], sources[0]
        handles = {'color_mapper': self.handles['color_mapper'],
                   'source': self.handles['source'],
                   'cds': self.handles['source'],
                   'main_source': main_source}
        axis = 'y' if self.invert_axes else 'x'
        callback = self._callback.format(axis=axis)
        if box_select.callback:
            box_select.callback.code += callback
            box_select.callback.args.update(handles)
        else:
            box_select.callback = CustomJS(args=handles, code=callback)
        return ret