Detect a faceted spec — either the top-level facet operator or the
encoding.row / encoding.column shorthand on a unit / layered spec —
and normalise it into a FacetDescriptor.
Returns null when the spec is not faceted (or is malformed, e.g.
facet without spec).
Detect a faceted spec — either the top-level
facetoperator or theencoding.row/encoding.columnshorthand on a unit / layered spec — and normalise it into a FacetDescriptor.Returns
nullwhen the spec is not faceted (or is malformed, e.g.facetwithoutspec).