Allow categorical aggregations in charts

When displaying data in charts, there's no way to display, for example, the distinct count of a categorical column as a measure. A workaround is to compute this in a group recipe, but it'd be much easier to throw ad-hoc charts together if categorical aggregations could be computed directly from within the charts tab. Optimally, even fields treated as numerical could be converted to be treated as categorical measures that can then be counted. I can't think of anything other than distinct count that would be useful to add here, since count of records is already an option.