Sets a computable value for a Field item in the Report section. This property is analogous to editing or entering a formula for a selected field in the Expression bar. The value specified instructs the application from where to retrieve information.
This example shows how to change the chart axis type to the X-axis category:
ActiveDocument.Sections["Sales Report"].ReportHeader.Fields["ReportName Field"].Formula = "ReportName() + ' ' + new Date()"