PHPExel LineChart: How can I display empty cell as a gap and not as 0 value - phpexcel

Question

I would like to generate a LineChart with PHPExcel, where empty cells are not displayed as 0 values but displayed as a gap.
In my code I tried in the constructor of PHPExcelChart a lot of different values for the argument "displayBlanksAs", such like ' ' or DISPLAY_BLANKS_AS_GAP or 'X' or NULL, etc. But nothing worked. The empty cell is always displayed as 0 not as a gap.
Is there anyone, who has an idea?Many thanks in advance!
Here my PHP-Code:
<?php
...
$dataSeriesValues = array(
new PHPExcel_Chart_DataSeriesValues('Number', 'Worksheet!$D$58:$Q$58', NULL, 14),);
$series = new PHPExcel_Chart_DataSeries(
PHPExcel_Chart_DataSeries::TYPE_LINECHART, // plotType
PHPExcel_Chart_DataSeries::GROUPING_STACKED, // plotGrouping
range(0, count($dataSeriesValues)-1), // plotOrder
NULL, // plotLabel
NULL, // plotCategory
$dataSeriesValues // plotValues);
// Set the series in the plot area
$plotArea = new PHPExcel_Chart_PlotArea(NULL, array($series));
// Create the Chart
$chart = new PHPExcel_Chart(
'chart1', // name
NULL, // title
NULL, // legend
$plotArea, // plotArea
true, // plotVisibleOnly
DISPLAY_BLANKS_AS_GAP, // displayBlanksAs => here I tried ' ',NULL,'X', etc., empty cells are always displayed as 0, not as a gap
NULL, // xAxisLabel
NULL // yAxisLabel
);
// Set the position where the chart should appear in the worksheet
$chart->setTopLeftPosition('R58');
$chart->setBottomRightPosition('X58');
$objPHPExcel->getActiveSheet()->addChart($chart);
?>

Answer 1

I found the answer myself:
I had to change in the constructor of PHPExcel_Chart_DataSeries the plotGrouping - parameter from GROUPING_STACKED to GROUPING_STANDARD:
$series = new PHPExcel_Chart_DataSeries(
PHPExcel_Chart_DataSeries::TYPE_LINECHART, // plotType
PHPExcel_Chart_DataSeries::GROUPING_STANDARD, // plotGrouping
range(0, count($dataSeriesValues)-1), // plotOrder
NULL, // plotLabel
NULL, // plotCategory
$dataSeriesValues // plotValues);
Then the empty cells are displayed as gaps.