I'm afraid I'm missing something here. Clear means delete all selections, meaning less restricted data or more general data. So the natural behavior I'd expect in a chart when clearing selections is to get exactly more rows, rather than less.
Say you have a "ItemNo" field, used in a listbox, and a
expression in a text object, that displays all customers that have bought any item. If you select one value in the ItemNo listbox, the Count() will show those customers that have bought only this product. If you select two items, the Count() will return more customers, those who have bought any of those items. When you clear selections, the Count() must return all customers, because it's counting customers that have bought all possible items (in white in the listbox).
Can you please elaborate a bit further about your specific issue?