If you want to be able to select a dimension from a listbox with fields then look at the top chart in attached qvw. If you want the dimension in your chart to change on selecting values in a field listbox then look at the bottom chart.
comm72990.qvw 208.5 K
The second table helps a lot. However, it satisfies the second half of the requirement. By default the dimension should be my Drill down group and everyone should be able to drill down as ususal.
Then when someone selects from the list boxes, the drill down dimension should go away and use the calculated dimension that you are using in the second chart.