How to toggle visibility of a column group

I have a table of project time recordings that I am displaying in a matrix. The data source returns:

        ProjectName
        ColleagueName
        Date
        Time
        Cost

similar to:

I've created two row groups:

        ProjectName
        ColleagueName - which is a subgroup of ProjectName

I have one column group:

        Date

Toggling visibility of the rows at the project level is working fine. However I'd like to toggle the visibility of the "Dates"columns and I can't get this to work. If I set the visibility of the "Date" column group to "Hide", they're hidden as I want. However I'd like to be able to toggle this in the same way as I can for the "ColleagueName".

When I try this, I have to enter a report item on which to toggle. No matter which field I select, I get an error similar to:

(where the toggle item displayed depends on which report item I've selected).

Can anyone suggest how I can achieve this functionality?

Thanks.