So if I understand you correctly, you want to - each week - duplicate the sheet with the graph, then connect the newly created sheet with a new excel sheet?
I think the only way to do that is to duplicate the data connection as well. Duplicate the connection (rightclick => duplicate), then edit the connection to point to the second sheet, then you can change the data source of your new sheet by rightclicking the data source and choosing replace data source.
But it seems this could be a very complicated way that gets a lot of data in one file... If each sheet is a new data source with it's own connection.... is that handy?
Is there not a way to get all your data in one sheet and just select the most recent info?