Add AggregateTransform to a vega-lite spec.
An input vega-lite spec
Array of objects that define fields to aggregate.
The data fields to group by. If not specified, a single group containing all data objects will be used.
Directly input an object, rather than creating one via the other arguments. Should not be used in conjunction with the other arguments other than
spec. Objects can be of type: AggregateTransform