Charts don't update when data is changed, why not?

Can someone help? My Excel has somehow ended up in a weird mode where
the charts don't change when the underlying data changes. The only way
to get a current chart is to close Excel and reopen it.

For example, if I were to construct a chart using two columns of
numbers, it will plot correctly. But if I go back and place a negative
sign in front of every y-axis datum, the chart won't change until I
close the file and reopen it.

I assume I have a config issue somewhere, but I can't find it. Can
anyone help? Thanks in advance.
/frank

I suppose it would be helpful if I mentioned that I'm running Excel
2003 on an XP-SP2 box.