fix graph data

Author: troosan

src/PhpWord/Writer/Word2007/Part/Chart.php

@@ -236,7 +236,7 @@ private function writeSeriesItem(XMLWriter $xmlWriter, $type, $values)
             $xmlWriter->startElement('c:pt');
             $xmlWriter->writeAttribute('idx', $index);
             $xmlWriter->startElement('c:v');
-            $this->writeText($value);
+            $xmlWriter->writeText($value);
             $xmlWriter->endElement(); // c:v
             $xmlWriter->endElement(); // c:pt
             $index++;