Thanks for your posting and using Aspose.Cells.
If you have formulas in your workbook then before you do anything, you should first call Workbook.calculateFormula() method and if you somehow changes your formula then before saving you should call Workbook.calculateFormula(). Means, you should call any time when you want to calculate the formulas.
Here is a sample code how to call this method.
Workbook wb = new Workbook(sampleFile);
//Now export your graph to picture