Add select transforms.

[mbostock]

Introduces new transforms for pulling out individual values from the data, while being z- and facet-aware.

[mbostock]

I thought about naming these Plot.selectFirst, Plot.selectLast, Plot.selectMinX etc. Let me know if you prefer that additional verbosity.

[Fil]

I guess Plot.selectFirst etc. might help discoverability.

[mbostock]

Okay, I adopted the longer names.

[mbostock]

I’m feeling pretty good about this and will merge but let me know if you have any objections or suggestions! Thanks.

[Fil]

It's working great, and I like the way it anticipates multiple values. To test this out and make it more extensible I've tried to export Plot.select.

Then for example, I can select a few values to label a line chart:

    Plot.text(data, {
      transform: Plot.select(I => d3.range(50, I.length, 100), null, {}),
      x: "date",
      y: "value",
   })

the null and {} are unfortunate in this case (since I don't need any channel to work with), so maybe we could make the third argument optional.

I could also detect n local maxima, etc., by adding my own functions.

If we do export it, the function should probably be called differently, to avoid confusion with d3.select. Note that it's a filter (#138), in a somewhat raw form. A signature like Plot.selectFilter((I, V) => …, {...options, key: accessor}) might be easier to use than (I, V) => …, "key", options.