quantile and threshold color scales

[Fil]

color: {type: "threshold", domain: [30, 50], range: ["red", "green", "blue"]}
closes #372

color: {type: "quantile", scheme: "blues", quantiles: 7}
closes #373

TODO

• see bug below with ordinal schemes

(still needs a few test plots)

[Fil]

we'll use the test plot in #419

[Fil]

I've added a build in https://observablehq.com/@fil/quantile-threshold-shaperendering-415-419

Here's a bug though: Threshold/quantile scales should accept ordinal schemes.

Plot.dot(data, { x: "bill_length", y: "species", fill: "body_mass" }).plot({
  color: {
    type: "threshold",
    domain: [2000, 3000],
    scheme: "dark2"
  }
})

gives TypeError: ordinalScheme(scheme === undefined ? "rdylbu" : scheme) is not a function. (In 'ordinalScheme(scheme === undefined ? "rdylbu" : scheme)({length: domain.length + 1})', 'ordinalScheme(scheme === undefined ? "rdylbu" : scheme)' is an instance of Array)

[mbostock]

For the diverging schemes (scheme11’s) you don’t get very good results with n = 2: you get the first two (red and gray in RdBu or red and yellow in RdYlBu).

What you want is probably the outer two (red and blue).

Maybe we can fix this in scheme11 for the n = 2 case.

plot/src/scales/schemes.js

Line 153 in 4286100

n = n > 3 ? Math.floor(n) : 3;

[mbostock]

The reason for the TypeError is that sometimes ordinalScheme returns an array (for the ordinal schemes) while other times it returns a function that will later be converted into an array when the domain is known:

plot/src/scales/ordinal.js

Lines 16 to 17 in 96d2f3f

	// If the range is specified as a function, pass it the domain.
	if (typeof range === "function") range = range(domain);

So we either need to make sure ordinalScheme always returns a function, or we need to handle a returned array when we call ordinalScheme. I can fix.