code: private function writeBubbles($plotSeriesValues, $objWriter)
      {
          if ($plotSeriesValues === null) {
              return;
          }
  
          $objWriter->startElement('c:bubbleSize');
          $objWriter->startElement('c:numLit');
  
          $objWriter->startElement('c:formatCode');
          $objWriter->writeRawData('General');
          $objWriter->endElement();
  
          $objWriter->startElement('c:ptCount');
          $objWriter->writeAttribute('val', $plotSeriesValues->getPointCount());
          $objWriter->endElement();
  
          $dataValues = $plotSeriesValues->getDataValues();
          if (!empty($dataValues)) {
              if (is_array($dataValues)) {
                  foreach ($dataValues as $plotSeriesKey => $plotSeriesValue) {
                      $objWriter->startElement('c:pt');
                      $objWriter->writeAttribute('idx', $plotSeriesKey);
                      $objWriter->startElement('c:v');
                      $objWriter->writeRawData(1);
                      $objWriter->endElement();
                      $objWriter->endElement();
                  }
              }
          }
  
          $objWriter->endElement();
          $objWriter->endElement();
  
          $objWriter->startElement('c:bubble3D');
          $objWriter->writeAttribute('val', 0);
          $objWriter->endElement();
      }
docs: Write Bubble Chart Details.
  
  @param DataSeriesValues $plotSeriesValues
  @param XMLWriter $objWriter XML Writer