Derived tip when grouping primitive values shows the wrong value

[mbostock]

Ref. https://observablehq.com/d/42b72061ec8200e4

Screen.Recording.2023-05-30.at.11.59.56.AM.mov

It appears to have something to do with the default identity title.

Plot.plot({
  height: 80,
  x: {type: "band"},
  marks: [
    Plot.barY(chars, Plot.groupX({y: "count"}, {x: (d) => d, tip: true}))
  ]
})

If you change the definition to avoid this behavior, the tip shows the correct value:

Plot.plot({
  height: 80,
  x: {type: "band"},
  marks: [
    Plot.barY(chars.map((d) => ({char: d})), Plot.groupX({y: "count"}, {x: "char", tip: true}))
  ]
})

This feels related to the discussion we were having in #1649 on channel values supplied as arrays of values, and transforms breaking that association. I’m guessing that the isTextual heuristic is wrong here (related #1646): the heuristic is being applied to the mark’s raw data, but that data is subsequently transformed (by the group transform), and so maybe the heuristic shouldn’t apply?

[mbostock]

These do the right thing:

Plot.plot({
  height: 80,
  x: {type: "band"},
  marks: [
    Plot.barY(chars, Plot.groupX({y: "count"}, {x: (d) => d, title: (d) => d, tip: true}))
  ]
})

Plot.plot({
  height: 80,
  x: {type: "band"},
  marks: [
    Plot.barY(chars, Plot.groupX({y: "count", title: (d) => d}, {x: (d) => d, tip: true}))
  ]
})

Plot.plot({
  height: 80,
  x: {type: "band"},
  marks: [
    Plot.barY(chars, Plot.groupX({y: "count"}, {x: (d) => d, title: Plot.identity, tip: true}))
  ]
})

I think the fix is pretty simple—we just need to opt-out of the additional defaults when constructing the derived tip mark. We never want to compute “extra” channels when deriving a tip.