Distinguish select from selectAll

The select transform, when you provide a function, is really “selectAll”: it can select multiple elements in each series. Whereas all the built-in named selectors only select a single element. This makes it a little confusing. For example, using a built-in selector, you’d say:

Plot.selectFirst({x: "Date", y: "Close"})

Or equivalently:

Plot.select("first", {x: "Date", y: "Close"})

But if you implemented this as a function with the obvious interpretation, it will crash!

Plot.select((I) => I[0], {x: "Date", y: "Close"}) // 💥

You need to return a single-element array instead:

Plot.select((I) => [I[0]], {x: "Date", y: "Close"})

It’d be nice if we the select transform’s selector expected you to return a single index, and we provided a separate selectAll transform for selecting multiple things (within each series). Then the above would work, and if you want to select multiple values, you can use selectAll:

Plot.selectAll((I) => [I[0], I[I.length - 1]], {x: "Date", y: "Close"})

Start at the select transform API described in the issue and compare the built-in selectFirst/select behavior with function selectors. Check how selectors are interpreted per series, then verify that single-index select and multi-index selectAll match the examples and intended distinction.