`resolve_scaled` followed by `facet` gives an inscrutable error `Unrecognized scale name: "child_layer_0_y"`

[rambip]

Bug Description

Goal: create a layered chart, facet it, and resolve axis independently. If the resolve_scale is not at the right place, it will fail with an error that gives no clue what the problem is.

Note that I used vega-altair to generate the expression, but I think that both altair and vega-lite have a part of responsability in the issue.

First attempt:

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.20.1.json",
  "columns": 6,
  "data": {
    "values": [
      {"country": 0, "v1": 100, "v2": 4, "date": 0},
      {"country": 0, "v1": 200, "v2": 7, "date": 1},
      {"country": 0, "v1": 300, "v2": 9, "date": 2},
      {"country": 1, "v1": 400, "v2": 6, "date": 0},
      {"country": 1, "v1": 500, "v2": 7, "date": 1},
      {"country": 1, "v1": 600, "v2": 5, "date": 2}
    ]
  },
  "facet": {
    "field": "country",
    "type": "nominal"
  },
  "spec": {
    "layer": [
      {
        "encoding": {
          "x": {
            "field": "date",
            "type": "temporal"
          },
          "y": {
            "field": "v1",
            "type": "quantitative"
          }
        },
        "mark": {
          "type": "line",
          "stroke": "red",
        }
      },
      {
        "encoding": {
          "x": {
            "field": "date",
            "type": "temporal"
          },
          "y": {
            "field": "v2",
            "type": "quantitative"
          }
        },
        "mark": {
          "type": "line"
        }
      }
    ],
    "resolve": {
      "scale": {
        "y": "independent"
      }
    }
  }
}

I get the error:

Unrecognized scale name: "child_layer_0_y"

Second attempt. The difference is that resolve is specified at the root and not in the layers.

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.20.1.json",
  "columns": 6,
  "data": {
    "values": [
      {"country": 0, "v1": 100, "v2": 4, "date": 0},
      {"country": 0, "v1": 200, "v2": 7, "date": 1},
      {"country": 0, "v1": 300, "v2": 9, "date": 2},
      {"country": 1, "v1": 400, "v2": 6, "date": 0},
      {"country": 1, "v1": 500, "v2": 7, "date": 1},
      {"country": 1, "v1": 600, "v2": 5, "date": 2}
    ]
  },
  "facet": {
    "field": "country",
    "type": "nominal"
  },
  "resolve": {
      "scale": {
        "y": "independent"
      }
    },
  "spec": {
    "layer": [
      {
        "encoding": {
          "x": {
            "field": "date",
            "type": "temporal"
          },
          "y": {
            "field": "v1",
            "type": "quantitative"
          }
        },
        "mark": {
          "type": "line",
          "stroke": "red",
        }
      },
      {
        "encoding": {
          "x": {
            "field": "date",
            "type": "temporal"
          },
          "y": {
            "field": "v2",
            "type": "quantitative"
          }
        },
        "mark": {
          "type": "line"
        }
      }
    ],
  }
}

This time, it did not return an error:

Expected behaviour

The first attempt should either work, or either indicate that resolve is not set in the right place.

[domoritz]

What do you mean by it doesn't work in the second case? The y axis is independent. Or do you mean you want dual axis chart? Then that's the first spec which as you noticed has a bug and breaks. It's supposed to work.

[rambip]

What do you mean by it doesn't work in the second case? The y axis is independent. Or do you mean you want dual axis chart? Then that's the first spec which as you noticed has a bug and breaks. It's supposed to work.

I must have mixed the codes I used to reproduce the bug. You're right, the second version works fine.