public function getRenderer($renderer_id)
    {
        if (array_key_exists($renderer_id, $this->renderers_by_id)) {
            return $this->renderers_by_id[$renderer_id];
        }

        throw new SwatObjectNotFoundException(
            "Cell renderer with an id of '{$renderer_id}' not found.",
            0,
            $renderer_id
        );
    }