generalize Plot.select

[Fil]

demo: https://observablehq.com/@observablehq/plot-select-generalize-656

closes #515

select a sample:

select min and max of a color:

[mbostock]

How about this?

Plot.select({x: "min"}, options)

And we could throw an error if you pass an object with more than one (or less than one) key as the first argument.

[mbostock]

On the topic of having a selector interface {select(I, V)} instead of just a function(I, V), so that we could potentially have a more convenient shorthand form for functions, I couldn’t think of a better shorthand for functions in this case, so I’m going to punt on that even though it’s a little different than what we did for reduce and map. Unlike, say, the filter transform, the select transform operates on a group of data at a time, so we need a way to identify a subset of that data, and a subset of the index feels like the most natural form for that.