Intervals for binning

[mbostock]

When you have data with a certain regularity, e.g. integers, it’d be nice if you could indicate that to the bin transform. This would allow it to “nice” the domain to contain that interval, and compute the appropriate thresholds. For example, say you have hourly data, you might want to specify the interval option instead of thresholds:

Plot.plot({
  marks: [
    Plot.rectY(data, Plot.binX({y: "count"}, {
      x: d => d.timestamp.getHours(),
      domain: [0, 24],
      interval: 1
    })),
    Plot.ruleY([0])
  ]
})

Previously #733.