Hi Brian,
I tried to reproduce the problem using this attached sample but was not able to see the problem.
sample :
42610TemplateChart.zip
The Sample will
1)Open a template file having a formula and a pie chart using code,wb = ExcelUtils.Open(fileName).
2)Edit the template file using XlsIO,by writing a value calculated using Calculate into the cell "B4" and insert a new pie chart to the template.
Please take a look at the attachment.Let me know if this solves your issue.
If not Could you please send us a template file in which you are having problem.
Please let me know if you need any further assistance.
Thanks,
Bharath.