how to get the sum of variables within a category instead of the count of rows?

[v4lue4dded]

This project looks amazing as a backend for interactive visualizations, but I can't seem to fingure out how to get a sum of a variable in a loaded table.

Say for example the sum over the Distance and the Delay variables in the Airline on-time performance dataset to get the total distance flown or total delay experienced for each date.
https://dig.cmu.edu/falcon-vis/crossfilter-duckdb/

In crossfilter this was done using the reduce function and passing it reduceAdd, reduceRemove, and reduceInital sub functions.

How is it implemented here? Can you maybe add an example to the readme?