Get plotVisOnly and dispBlanksAs props from chart in Xlsx Chart Writer.

These chart properties where hardcoded in the Xlsx writer, ignoring the
values passed to the \PhpOffice\PhpSpreadsheet\Chart\Chart class constructor.

I also changed the default $displayBlanksAs value to 'gap' to match this
document:
https://docs.microsoft.com/en-us/openspecs/office_standards/ms-oi29500/1942c028-3c4e-4dd9-a379-b12a9b7914df

Closes #1266

CHANGELOG.md

@@ -22,6 +22,7 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 - Do not confuse defined names and cell refs [#1263](https://github.com/PHPOffice/PhpSpreadsheet/pull/1263)
 - XLSX reader/writer keep decimal for floats with a zero decimal part [#1262](https://github.com/PHPOffice/PhpSpreadsheet/pull/1262)
 - ODS writer prevent invalid numeric value if locale decimal separator is comma [#1268](https://github.com/PHPOffice/PhpSpreadsheet/pull/1268)
+- Xlsx writer actually writes plotVisOnly and dispBlanksAs from chart properties [#1266](https://github.com/PHPOffice/PhpSpreadsheet/pull/1266)
 
 ## [1.10.0] - 2019-11-18
 

src/PhpSpreadsheet/Chart/Chart.php

@@ -156,7 +156,7 @@ class Chart
      * @param null|GridLines $majorGridlines
      * @param null|GridLines $minorGridlines
      */
-    public function __construct($name, Title $title = null, Legend $legend = null, PlotArea $plotArea = null, $plotVisibleOnly = true, $displayBlanksAs = '0', Title $xAxisLabel = null, Title $yAxisLabel = null, Axis $xAxis = null, Axis $yAxis = null, GridLines $majorGridlines = null, GridLines $minorGridlines = null)
+    public function __construct($name, Title $title = null, Legend $legend = null, PlotArea $plotArea = null, $plotVisibleOnly = true, $displayBlanksAs = 'gap', Title $xAxisLabel = null, Title $yAxisLabel = null, Axis $xAxis = null, Axis $yAxis = null, GridLines $majorGridlines = null, GridLines $minorGridlines = null)
     {
         $this->name = $name;
         $this->title = $title;

src/PhpSpreadsheet/Writer/Xlsx/Chart.php

@@ -83,11 +83,11 @@ public function writeChart(\PhpOffice\PhpSpreadsheet\Chart\Chart $pChart, $calcu
         $this->writeLegend($objWriter, $pChart->getLegend());
 
         $objWriter->startElement('c:plotVisOnly');
-        $objWriter->writeAttribute('val', 1);
+        $objWriter->writeAttribute('val', (int) $pChart->getPlotVisibleOnly());
         $objWriter->endElement();
 
         $objWriter->startElement('c:dispBlanksAs');
-        $objWriter->writeAttribute('val', 'gap');
+        $objWriter->writeAttribute('val', $pChart->getDisplayBlanksAs());
         $objWriter->endElement();
 
         $objWriter->startElement('c:showDLblsOverMax');