sort aliases

[mbostock]

Fixes #769. Introduces a concept of channel aliases and derived channels when sorting. For example, if you request to sort on “y”, but there is no y channel, it will fallback to y2. The new “dx” and “dy” refer to implicitly derived channels representing |x2 - x1| and |y2 - y1| respectively. For these channels, reverse defaults to true (since they typically represent a length encoding rather than a position encoding).

Plot.plot({
  x: {
    tickFormat: "d",
    label: null
  },
  y: {
    label: "↑ Annual production (quads)"
  },
  color: {
    tickFormat: t => types.get(t),
    legend: true
  },
  marks: [
    Plot.rectY(energy, {x: "Year", interval: 1, y: "Value", fill: "Description", sort: {color: "dy"}}),
    Plot.ruleY([0])
  ]
})

[Fil]

Yes. Although I think it would be easier to memorize and explain if it were called width and height rather than dx and dy?