Auto mark isMonotonic should distinguish sorted within series

If data is monotonic in X or Y, the auto mark may draw a line. But we have a longstanding TODO in isMonotonic:

https://github.com/observablehq/plot/blob/ee5eff7844a48453a4c6239d9f11363edc0879c6/src/marks/auto.js#L246-L263

For example, this data:

const data = [
    {date: new Date(2020, 0, 1), value: 5, series: 1},
    {date: new Date(2020, 0, 1), value: 50, series: 2},
    {date: new Date(2020, 0, 1), value: 500, series: 3},
    {date: new Date(2020, 0, 2), value: 6, series: 1},
    {date: new Date(2020, 0, 2), value: 60, series: 2},
    {date: new Date(2020, 0, 2), value: 600, series: 3},
    {date: new Date(2020, 0, 3), value: 7, series: 1},
    {date: new Date(2020, 0, 3), value: 70, series: 2},
    {date: new Date(2020, 0, 3), value: 700, series: 3}
  ]
  return Plot.auto(data, {x: "date", y: "value"}).plot();

Produces a weird line:

If two points have the same value (so order === 0), should isMonotonic be false? There are fancier things we could do, like trying to infer a z channel, but…

(Idk if this is a bug or enhancement!)

Read src/marks/auto.js at lines 246-263, then reproduce the issue with the supplied Plot.auto data containing three series. Determine how equal or interleaved values should affect isMonotonic, and verify that the example no longer produces a misleading line when the behavior is corrected.