Pass the bin extent to reducers?

[Fil]

A typical example would be to have the title of each bar show the date extent in https://observablehq.com/@fil/plot-stargazer-history

    Plot.rectY(
      data,
      Plot.binX(
        {
          y: "count",
          title: (bin) => `${bin.x1}…${bin.x2}` // ⇐ something like this.
        },
        {
          x: "date",
          fill: "brown",
          thresholds: 100
        }
      )
    ),

[mbostock]

By analogy, we could pass the key to group reducers.