Error bars

[mbostock]

We should have an easier way to add error bars to a bar, as shown here using a rule and tick:

Plot.plot({
  marks: [
    Plot.barX(alphabet, {x: "frequency", y: "letter", sort: {y: "-x"}, fill: "steelblue"}),
    Plot.ruleY(alphabet, {x1: (d) => d.frequency * 0.9, x2: (d) => d.frequency * 1.1, y: "letter"}),
    Plot.tickX(alphabet, {x: (d) => d.frequency * 0.9, inset: 6, y: "letter"}),
    Plot.tickX(alphabet, {x: (d) => d.frequency * 1.1, inset: 6, y: "letter"}),
    Plot.ruleX([0])
  ]
})

Ref. #1868 (comment)