scaleQuantize doesn't work as expected with reversed domain

[patricksurry]

Using https://d3js.org/d3.v4.min.js:

fwd = d3.scaleQuantize().domain([0,1]).range([0,1,2])
bwd = d3.scaleQuantize().domain([1,0]).range([0,1,2])
pts = [0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1]
pts.map(fwd)
=> [0, 0, 0, 0, 1, 1, 1, 2, 2, 2, 2]
pts.map(bwd)
=> [0, 0, 0, 0, 2, 2, 2, 2, 2, 2, 2]

Other d3 scales with continuous domain seem to happily support the first endpoint greater than the second (super handy for swapping axes around etc), but scaleQuantize didn't do what I expected.

[mbostock]

Correct—the code currently assumes that the domain is in ascending order, and the behavior is undefined if it’s not. (It uses d3.bisect internally.)

There are other scales that require ascending domains, namely d3.scaleThreshold. (d3.scaleQuantile internally computes a sorted array of quantile thresholds from an unsorted domain, so it’s likewise not possible to have a “descending” quantile scale.)

That said it seems reasonable here to support a descending quantize scale. However, I’m not sure what quantize.invertExtent should return in this case. Should it return [x0, x1] where x0 < x1 for a descending domain, or x0 > x1? My intuition is the latter, so that the order of the array returned by invertExtent is consistent with the domain. But I’m not entirely sure.