I'm learning how to work with charts and came accross the following problem.

I have a report with a grouping and i want to add a chart in the group footer. The chart must only use records from within the group.
When i access the chart expert on the Data-tab i cannot choose the option 'Group'. It's always advanced.
Any idea what i have to do to enable this option ?
I'm using the 'bar' as chart type.

Thanx...