temporal tick format

[Fil]

When a point or band scale is based on time values, use a time format for ticks; the default time format is the multi-time format from utcScale, but can be overridden with tickFormat: "%a". Similarly, if the point or band scale is based on numeric (ie non-ordinal) values, tickFormat: "," is interpreted with d3.format.

closes #212

I'm sorry that this PR is not pretty; a part of the problem is that d3's original scaleBand and scalePoint aren't behaving like scaleLinear and scaleUtc (ie they don't have a tickFormat method). ((The main part though is just me not finding the style.))

As a result, with no specific settings in x we get:

Plot.plot({
  marks: [
    Plot.barY(sales, Plot.stackY({x: "date", y: "units", fill: "fruit"})),
    Plot.text(sales, Plot.stackY({x: "date", y: "units", text: "fruit" }))
  ]
});

and x: {tickFormat: "%a"} or whatever if we want to personalize the scale.

[mbostock]

We should use === undefined here; null should be interpreted as “don’t show any tick labels”.

Also, I think we want to use the same default date formatter as Observable Inputs’ Table (which is the same one that’s used by Observable’s inspector and d3-dsv):

https://github.com/observablehq/inputs/blob/853b17d0aada3f756df5a524f2852d9682e2b60a/src/format.js#L35-L46

The reason being: dates in an ordinal dimension, unlike ticks on a temporal scale, are not necessarily at regular intervals. The formatter I recommend will return a string that’s “as small as possible” without losing any information about what date is being shown.