code: public function writeDrawingRelationships(\PhpOffice\PhpSpreadsheet\Worksheet\Worksheet $pWorksheet, &$chartRef, $includeCharts = false)
      {
          // Create XML writer
          $objWriter = null;
          if ($this->getParentWriter()->getUseDiskCaching()) {
              $objWriter = new XMLWriter(XMLWriter::STORAGE_DISK, $this->getParentWriter()->getDiskCachingDirectory());
          } else {
              $objWriter = new XMLWriter(XMLWriter::STORAGE_MEMORY);
          }
  
          // XML header
          $objWriter->startDocument('1.0', 'UTF-8', 'yes');
  
          // Relationships
          $objWriter->startElement('Relationships');
          $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/package/2006/relationships');
  
          // Loop through images and write relationships
          $i = 1;
          $iterator = $pWorksheet->getDrawingCollection()->getIterator();
          while ($iterator->valid()) {
              if ($iterator->current() instanceof \PhpOffice\PhpSpreadsheet\Worksheet\Drawing
                  || $iterator->current() instanceof MemoryDrawing) {
                  // Write relationship for image drawing
                  $this->writeRelationship(
                      $objWriter,
                      $i,
                      'http://schemas.openxmlformats.org/officeDocument/2006/relationships/image',
                      '../media/' . str_replace(' ', '', $iterator->current()->getIndexedFilename())
                  );
              }
  
              $iterator->next();
              ++$i;
          }
  
          if ($includeCharts) {
              // Loop through charts and write relationships
              $chartCount = $pWorksheet->getChartCount();
              if ($chartCount > 0) {
                  for ($c = 0; $c < $chartCount; ++$c) {
                      $this->writeRelationship(
                          $objWriter,
                          $i++,
                          'http://schemas.openxmlformats.org/officeDocument/2006/relationships/chart',
                          '../charts/chart' . ++$chartRef . '.xml'
                      );
                  }
              }
          }
  
          $objWriter->endElement();
  
          return $objWriter->getData();
      }
docs: Write drawing relationships to XML format.
  
  @param \PhpOffice\PhpSpreadsheet\Worksheet\Worksheet $pWorksheet
  @param int &$chartRef Chart ID
  @param bool $includeCharts Flag indicating if we should write charts
  
  @throws WriterException
  
  @return string XML Output