code: private function writeGradientFill(XMLWriter $objWriter, Fill $pFill)
      {
          // fill
          $objWriter->startElement('fill');
  
          // gradientFill
          $objWriter->startElement('gradientFill');
          $objWriter->writeAttribute('type', $pFill->getFillType());
          $objWriter->writeAttribute('degree', $pFill->getRotation());
  
          // stop
          $objWriter->startElement('stop');
          $objWriter->writeAttribute('position', '0');
  
          // color
          $objWriter->startElement('color');
          $objWriter->writeAttribute('rgb', $pFill->getStartColor()->getARGB());
          $objWriter->endElement();
  
          $objWriter->endElement();
  
          // stop
          $objWriter->startElement('stop');
          $objWriter->writeAttribute('position', '1');
  
          // color
          $objWriter->startElement('color');
          $objWriter->writeAttribute('rgb', $pFill->getEndColor()->getARGB());
          $objWriter->endElement();
  
          $objWriter->endElement();
  
          $objWriter->endElement();
  
          $objWriter->endElement();
      }
docs: Write Gradient Fill.
  
  @param XMLWriter $objWriter XML Writer
  @param Fill $pFill Fill style