Reporting Services sparklines not updating with new formula

Hi,

I have a report that is showing sparklines. Originally, the tablix was showing the formula as Avg(x / y). However, I came to find that because of the way the data is already aggregated in the query, the correct way to calculate the value is Sum(x) / Sum(y).

When I make this change in the tablix, the sparkline doesn't update, it still reflects the value of Avg(x / y).

If I delete the sparkline and insert a new one, no data at all is displayed. I am not sure I understand this, although I suspect I added another child column to the tablix since I originally created the first sparklines.

(This is targeting SSRS 2008 R2 from VS 2010.)

How can I manually determine what data the sparklines should use?

Thanks,

SA.