The field to encode. For multi-level categories, this is the innermost level.
OptionallevelsFor multi-level categories, the fields of the outer levels, from the outermost level inward.
For example, a Year > Quarter axis has levels: ['year'] and field: 'quarter'.
OptionalscaleThe scale that maps the field's values, named by its id. Optional for channels without a
scale, such as text.
Maps one data field to a visual channel.