Extracting Graph from TCanvas in File

Hi there,
I'm not sure whether the subject is really descriptive. I'm trying to process a number of files which have all the same structure, i.e. it contains only a TCanvas, but the names vary from file to file. Is there a way of casting the canvas and getting their content? In principle I want to extract the TGraphError "Graph" stored. And I can easily access it assuming I know the name of my TCanvas, e.g. (assuming c1 is the name of the canvas):
TGraphErrors *g1 = c1->GetPrimitive("Graph");

Any Idea how to do this?
Thanks a lot,
Cheers,
Stephan

-- 
Stephan Zimmer
zimmerst_at_gmail.com