I have noticed that this example is currently throwing the following error when opening the produced Excel Spreadsheet - "We found a problem with some content in <filename>.xlsx. Do you want us to try to recover as much as we can? If you trust the source of this workbook click Yes".
In order to fix this I have set the removed the PlotGrouping setting (currently PHPExcel_Chart_DataSeries::GROUPING_STANDARD) to NULL and it appears to work.