FacetEncodingFieldDef
case class FacetEncodingFieldDef(aggregate: Option[Aggregate], align: Option[VegaLiteDslAlign], bandPosition: Option[Double], bin: Option[AngleBin], bounds: Option[Bounds], center: Option[Center], columns: Option[Double], field: Option[Field], header: Option[Header], sort: Option[SortArray], spacing: Option[Spacing], timeUnit: Option[TimeUnitUnion], title: Option[LegendTitle], `type`: Option[StandardType])
A field definition for the (flexible) facet of trellis plots.
If either row or column is specified, this channel will be ignored.