Group by in Data Visualization presents error "Selected field is invalid"

[Greg Patrick]

I'm trying to create a Data Visualization, in particular a Pivot Table, that shows me a list of people who have raised requests of a particular type. I want to be able to capture in the Pivot Table a date from a variable that was captured on the form. However, when I attempt to do this, I get the "Selected field is invalid" error. Is the issue that you can't use variables for the purpose of Group By?

[Greg Patrick]

Further playing around I've worked out that I can use some variables in a Pivot Table, but not all. Is there some kind of limitation on what types of variables can be utilised in this manner?

[BurtCrepeault]

I also have this problem under Xanadu, I can't seem to be able to use date fileds in Group by.