It would appear you are looking for VBA code to do the needful. If that is the case, I suspect you will hit a double-whammy, as it were.
First, of all, accessing embedded/linked objects is tricky. Sometimes very tricky. For an intro see the incomplete
Case Study – Embedded and Linked Objects
http://www.tushar-mehta.com/excel/vba/vba-embedded or linked objects.htm
One of the more frustrating things I find is that sometimes information about an object is not available until you activate it at least once. Doing that through code is less than easy.
The other part of the whammy is that it is not easy to access information about the existing chart series. Of course, if all you want to do is overwrite the existing series with new range information, that should be easy. Just use the macro recorder in XL to get the necessary code. But, if you have to analyze and act on the existing series information, that's much more difficult. You can get started with John Walkenbach's
http://www.j-walk.com/ss/excel/tips/tip83.htm
Sorry mate, just one more thing...
If I want to have several PowerPoint presentations created from different graphs, and I want to make them all from one template PowerPoint presentation, how can I do that? Basically I want to copy and paste the PowerPoint file I have at the moment, and change the graph that the link references to on the paste versions but I can't seem to find the code which references the link. Any help appreciated
Thanks
Dan