NonArgAggregateOp

An aggregate operation to perform on the field prior to sorting (e.g., "count", "mean" and "median"). An aggregation is required when there are multiple values of the sort field for each encoded data field. The input data objects will be aggregated, grouped by the encoded data field.

For a full list of operations, please see the documentation for aggregate.

Default value: "sum" for stacked plots. Otherwise, "min".