Support gradientLength for `quantile` and `quantize` color scales about vega-lite HOT 4 OPEN

From a code dive, it looks like discrete gradient legend is already reading the length param. However, it is not having a (visible) effect on the height of the legend.

This FR could be reclassified as a bugfix if the resolution is to wire the existing gradientLegend param into the discrete gradient legend.

https://github.com/vega/vega/blob/8a7f2f233b7fec430e52fbe19a544c1f290be591/packages/vega-parser/src/parsers/guides/legend-gradient-discrete.js#L13

from vega-lite.

Yeah, sounds like a bug fix.

from vega-lite.

After running local tests (I hardcoded a value into guide-util.js and checked the test specs for legends-discrete, this is a bug for vega-lite rather than vega .

Vega respects the gradientLength if there was is one passed in. The Vega-Lite specs are losing gradientLength at compile time, not at runtime.

Custom size (120)	Default size (300)

from vega-lite.

I figured out a runtime fix, which I'll push shortly. The deeper conceptual question which might lead to a change back in vega is whether Discrete

https://github.com/vega/vega/blob/8a7f2f233b7fec430e52fbe19a544c1f290be591/packages/vega-parser/src/parsers/guides/constants.js#L25-L27

should be exported as a 3rd LegendType,

or if discrete should be classified as a special mode within gradient instead of as a mode under symbol.

from vega-lite.