Allow for using field/channel definition properties with datum

[joelostblom]

Bug Description

According to the docs, it should be possible to use datum together with field definition properties such as scale, axis, etc. When I create such a spec, the chart is rendered as expected, but datum is underlined with the warning "Property datum is not allowed":

This error/warning means that we can't create the chart from altair, since the schema validation fails. Since the docs imply that the intention is that this should be supported and the chart is working as expected, would it be possible to update the schema validation to not throw an error/warning in this case?

(I seem to recall using datum with scale in the past without issues, so maybe this is a regression? Or maybe I'm using it incorrectly and there is another way to combine datum with scale?)

[joelostblom]

It seems like for x and y encodings, there is no warning/error raised. So maybe it is just an issue with the color encoding. Here is a simple spec with x and color (note that only color gets the squiggly underline): Open the Chart in the Vega Editor

On the Altair side, this also shows up as an inconsistency between the two class methods when it comes to the new method based syntax:

[hydrosquall]

Assuming that the fact the runtime supports this is intentional (and not an accident), I think we need to take the DatumDef and at minimum extend it with the ScaleMixins

vega-lite/src/channeldef.ts

Lines 388 to 401 in d4492fe

	export interface DatumDef<
	F extends Field = string,
	V extends PrimitiveValue | DateTime | ExprRef | SignalRef = PrimitiveValue | DateTime | ExprRef | SignalRef,
	> extends Partial<TypeMixins<Type>>,
	BandMixins,
	TitleMixins {
	/**
	* A constant value in data domain.
	*/
	datum?: F extends RepeatRef ? V | RepeatRef : V;
	// only apply Repeatref if field (F) can be RepeatRef
	// FIXME(https://github.com/microsoft/TypeScript/issues/37586):
	// `F extends RepeatRef` probably should be `RepeatRef extends F` but there is likely a bug in TS.
	}

I'm not sure if Axis/Sort/Impute/Stack are all worth pulling in too since those don't have a mixin and we haven't checked if each of those are actually supported at runtime, but at minimum Scale seems reasonable to pull in based on your example

Out of curiosity, what would be the use-case for using a scale with a singular datum when you could set the value directly instead, like this ?