Copied Graph changes values

I have a spreadsheet with four graphs on it. I want to copy them to a new
workbook, but when I do something strange happens.
I copy and pasted the worksheet and everything is fine. But when I close the
original workbook the first label going along the bottom of the graphs
changes from Dec 2008 to a numeric value (the other months along the bottom
are fine) and the numeric values on the bars change, but the graphs still
look the same other that that. By that I mean even though the values on the
individual bars changes the height of the graphs doesn’t.
Finally, when I open the original workbook again, everything returns to
normal.
This has only just started to happen. Can anybody point to why? We are using
Excel 2007.