Google Charts is just one potential renderer out of many. It can be used as other renderers such as jpGraph and graphpite by writing a chartRenderer wrapper - not dissimilar to the way we use the IOFactory for reading and writing workbooks at present.
$chartWriter = PHPExcel_ChartFactory::createRenderer($objPHPExcel->getChartCollection()->getChart('Chart 1'), 'GoogleCharts');
At the moment, I'm less concerned with the details of what wrappers we'll provide, as I am about how we make the renderer engine 'generic' so that new wrappers can be plugged-in subsequently.
More critical is being able to read chart definitions from existing workbooks, create new chart definitions through PHP, and write chart definitions to workbooks... and it's the whole combination that makes this a substantial sub-project within PHPExcel.