Add groupSort.

[mbostock]

For example, to sort varieties by descending median yield:

d3.groupSort(barley, g => -d3.median(g, d => d.yield), d => d.variety)

[Fil]

Concerning groupSort, could that be made to work with nested grouping?

It might be nice to write

d3.groupSort(penguins, v => -v.length, d => d.species, d => d.island)

and the API of groups/rollups would feel more consistent.

We can craft that by hand by grouping on compounded keys:

d3.sort(
    d3.rollup(penguins, v => v.length, d => [d.island, d.species].toString()),
    ([, value]) => -value
  )
  .map(([key, value]) => key.split(/,/)) // [ ["Biscoe", "Gentoo"], … ]

(the toString hack would not be needed if d3.rollup accepted arrays as keys, as in #185 (comment))

Concerning d3.medianSort and the other shorthands, I wonder if it's not too much: they're easy to write with groupSort, and having min, max, median seems incomplete (why not the mean, length, etc). It's all just syntactic sugar so more a question of taste than anything else, but it seems a bit too sugary to me :)

[mbostock]

I don’t think nested grouping will work, at least as is — what would the comparator compare? If the comparator is passed both the leaf groups and the nested groups, the comparator will have to do some introspection to figure out what it’s comparing. If the comparator is passed the leaf groups and flattened nested groups, then it’ll work, but it won’t be very efficient and it’ll add some complexity for a use case I’m not confident is very common.

You can do the compound key thing with d3.groupSort directly:

d3.groupSort(penguins, v => -v.length, d => JSON.stringify([d.species, d.island])).map(JSON.parse)

I feel like the medianSort et al. are common enough to want, and simple enough to support, that it’s worth the trouble. It does raise the question of why no sumSort, meanSort, countSort, though… I suppose we could always start with groupSort and then add the helpers later?

[mbostock]

Pruned this PR down to just groupSort, and put the helper methods in another branch in case we want to add them later.