Barchart: add MAX/MIN/AVG functions, not just COUNT.
This would give you a sort of pivot function. e.g. you have a set of values (v) and a function (f) and you want to check the distribution of f's output for v's input. This is mostly useful for checking formula.
(Aside: I've thought it would be a great feature to be able to look up all the values that go into an expression and the expression result. This could possibly be implemented as a link to the report page with an expression in the query string. We could link to this from the survey definition editor.)