protected function displayRenderers($row)
    {
        if ($this->group_by === null) {
            throw new SwatException("Attribute 'group_by' must be set.");
        }

        $group_by = $this->group_by;

        // only display the group header if the value of the group-by field has
        // changed
        if (!$this->isEqual($this->header_current, $row->$group_by)) {
            $this->resetSubGroups();
            $this->displayGroupHeader($row);
            $this->header_current = $row->$group_by;
        }
    }