You think right, it can't be done. The List element is for record-by-record data access, and there's no way for it to consider more than one row at a time.
Working solely within a Dashboard, you can get this sort of thing to work in a Serial Chart, or in a series of Indicators. To truly get a plain list with aggregated data, you'd need to go into some custom development. The actual process of submitting a query to a Feature Service and getting back grouped values is quite simple, but working that into a web app or dashboard-like webpage takes a bit more work.
For an alternate approach, you can also accomplish this by introducing another layer. I've got a couple dashboards where I have a separate standalone table for the aggregated data that automatically re-calculates every hour.
- Josh Carlson
Kendall County GIS