Support gradientLength for quantile and quantize color scales

[hydrosquall]

Motivation

For continuous (linear) scales on a color encoding, the gradientLength config can be used to control legend length.

Standard	With custom config

Unfortunately, this property has no effect when using discrete color scales, like quantile or quantize . As far as I could tell, the symbol config options don't control height either.

This can lead to legends that are much taller than their graph (see below: the gradientLength param is set, but it appears to have no effect.)

Quantize	Quantile

Proposed solution

• Add at least 1 configuration option for discrete color scales, perhaps setting the overall legend height (since the quantile mode won't have even tick sizes), in the direction of discreteColorLegendLength .
• If this property is added for Vega first, it will have to be propagated up to Vega-Lite separately.

Related PRs

• While searching for past legend issues, I found
  • Correctly Support Quantile, Quantize, Threshold Scales #1092 marked the introduction of quantile/quantize/threshold scales
  • vega/vega@ddd2bbf introduced discrete gradient legend

[hydrosquall]

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

[domoritz]

Yeah, sounds like a bug fix.

[hydrosquall]

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)

[hydrosquall]

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.