Pivot Chart seems to show "old" data - how do I make it update?

Pivot Chart seems to show "old" data - how do I make it update?

I wonder how to make a Pivot Chart "update" its data contents.

Here is the background:

I need to report a subset of data that is so complicated that it is easier to make a calculation and put the result in a "reporting form", and then display the data from that form.  So, I get data from the "original form" GRANTS, make some calculations and put the result in the "reporting form" PERIOD. 

Now I can watch the result in a list called LIST, which works fine. The list LIST shows that form PERIOD contains exactly the data it should contain. So far so good.

I have also made a pivot chart called CHART to display the data from PERIOD in. The first time I look at CHART, it looks fine. The data in CHART is the same as in LIST, as it should.

But... here comes the problem:

If I clear out PERIOD, make new calculations and put the result into PERIOD, the LIST shows the new data just fine. But CHART seems to still contain the old data from the previous calculation! In other word, the pivot chart CHART doesn't seem to automatically update to show the latest version of the underlying form PERIOD. 

How do I make CHART use the latest data?