public function insert_bitmap($row, $col, $bitmap, $x = 0, $y = 0, $scale_x = 1, $scale_y = 1) {
        $objDrawing = new PHPExcel_Worksheet_Drawing();
        $objDrawing->setPath($bitmap);
        $objDrawing->setCoordinates(PHPExcel_Cell::stringFromColumnIndex($col) . ($row+1));
        $objDrawing->setOffsetX($x);
        $objDrawing->setOffsetY($y);
        $objDrawing->setWorksheet($this->worksheet);
        if ($scale_x != 1) {
            $objDrawing->setResizeProportional(false);
            $objDrawing->getWidth($objDrawing->getWidth()*$scale_x);
        }
        if ($scale_y != 1) {
            $objDrawing->setResizeProportional(false);
            $objDrawing->setHeight($objDrawing->getHeight()*$scale_y);
        }
    }