Enhancement Description

aggregate_params was added to vega aggregation transforms in vega/vega#3686, allowing exponential moving average. Can we add a way to add these aggregation_params into vega-lite aggregation transforms?

I'm new to vega and vega-lite, but, looking at the code, I think I'd be able to help in adding this into vega-lite.

Checklist

• I checked for duplicate issues.