Hi Paul,
In our both WPF and Web Report Designer we need to set the visibility for Tablix member as shown in below steps.
Step 1: Click the advanced mode for column grouping as shown in below snap.
Web Designer:
WPF Designer:
Step 2: Set the Visibility for Tablix member when clicking the column as shown in below snap.
Web Designer:
WPF Designer:
Regards,
Vinoth S.