Well, first you have to patch the code...
It's not supported in PHPExcel. The XML for graphs in Excel 2007 is surprisingly relatively straightforward, create a few graphs in Excel, unzip the files and take a look at what's different between them. You'll need to add extra properties to the appropriate
PHPExcel class and update the readers and writers to use them. I've been meaning to write and submit a patch for this for sometime since it'd be really useful for me as well but have never got around to it I'm afraid.