PHPExcel图表刻度线之间的X轴

$objPHPExcel = new PHPExcel();
$objPHPExcel->getActiveSheet()->setTitle('Chart Example');
$objPHPExcel->getActiveSheet()->fromArray(
    array(
        array('', 'Category 1', 'Category 2', 'Category 3'),
        array('Value 1', 10, 20, 30),
        array('Value 2', 40, 50, 60),
        array('Value 3', 70, 80, 90),
    )
);

$chart = new PHPExcel_Chart(
    'chart1', // 图表的名称
    null, // 图表的标题
    null, // 图表的坐标轴标签
    $objPHPExcel->getActiveSheet(), // 图表所在的工作表对象
    false // 是否将图表嵌入到工作表中
);
$chart->setTopLeftPosition('A5');
$chart->setBottomRightPosition('H20');
$chart->setChartType(PHPExcel_Chart_DataSeries::TYPE_BARCHART);

$chart->setXAxisTickValues(array(
    new PHPExcel_Chart_DataSeriesValues('String', 'Chart Example!$B$1:$D$1', null, 3), // 刻度线之间的X轴间距
));

$objPHPExcel->getActiveSheet()->addChart($chart);
$objWriter = PHPExcel_IOFactory::createWriter($objPHPExcel, 'Excel2007');
$objWriter->save('chart_example.xlsx');