public function set_bottom($style) {
        if ($style == 1) {
            $this->format['borders']['bottom']['style'] = PHPExcel_Style_Border::BORDER_THIN;
        } else if ($style == 2) {
            $this->format['borders']['bottom']['style'] = PHPExcel_Style_Border::BORDER_THICK;
        } else {
            $this->format['borders']['bottom']['style'] = PHPExcel_Style_Border::BORDER_NONE;
        }
    }