ConstReadonlyColorScale: "colorScale"A gradient across the range's values.
ReadonlyDataBar: "dataBar"An in-cell bar proportional to the value.
ReadonlyIconSet: "iconSet"An in-cell icon chosen by which band the value falls in.
OOXML spells eighteen cfRule types; fifteen of them are a predicate plus a style and are
modeled as ICondition + IConditionalFormat.getStyle. These three are not
boolean tests at all — they read the whole range and render across it — so they get their own
shape. The remaining OOXML type names are an IO concern and live in io-xlsx.
The three rules that are a value→visual mapping rather than a predicate.