`Undefined data set name` error when using a shared `x` encoding in a faceted+layered plot

[gsurrel]

Bug Description

I am trying to show faceted bar charts with the bar name layered on top of them. I have a basic version that works well without the facet, but adding the facet, I get [Error] Undefined data set name: "data_1"

Specification with facet and layers

{
  "$schema": "https://vega.github.io/schema/vega-lite/v6.json",
  "data": {
    "values": [
      {
        "group": "2025",
        "class": "A",
        "value": 23,
        "deltaX": 322,
        "class_color": "#041042"
      },
      {
        "group": "2025",
        "class": "B",
        "value": 21.5,
        "deltaX": 313,
        "class_color": "#7c31c0"
      },
      {
        "group": "2025",
        "class": "C",
        "value": 15,
        "deltaX": 275,
        "class_color": "#2652bc"
      }
    ]
  },
  "transform": [
    {
      "window": [
        {"field": "deltaX", "op": "max", "as": "deltaX_max"},
        {"field": "value", "op": "max", "as": "value_max"}
      ],
      "frame": [null, null]
    },
    {
      "calculate": "datum.deltaX / datum.deltaX_max * datum.value_max",
      "as": "value_published"
    },
    {"fold": ["value", "value_published"], "as": ["key", "value"]}
  ],
  "facet": { "row": {"field": "key"} },
  "spec": {
    "encoding": {
      "x": {"field": "value", "type": "quantitative", "title": null}, /* Triggers 'Undefined data set name: "data_1"' */
      "y": {"field": "group", "type": "nominal", "title": null},
      "yOffset": {
        "field": "class",
        "type": "nominal",
        "sort": {"field": "y", "order": "ascending"}
      }
    },
    "layer": [
      {
        "mark": {"type": "bar", "tooltip": true},
        "encoding": {
          "color": {
            "field": "class",
            "type": "nominal",
            "scale": {"range": {"field": "class_color"}},
            "title": null
          }
        }
      },
      {
        "mark": {
          "type": "text",
          "align": "right",
          "xOffset": -4,
          "color": "white"
        },
        "encoding": {
          "text": {"field": "class", "type": "nominal"}
        }
      }
    ]
  }
}

I can get a (imperfect) result by moving the .spec.encoding.x field down to .spec.layer[0].encoding.x. It does not work if I move it to the next (text) layer, nor if I duplicate it to the next (text) layer.

I tried multiple different variations, but none worked. I also had the same error message, but with another unknown varialbe ([Error] Undefined data set name: "facet"), but I did not save it.

Approximation of the desired result (without the facet, thus splitting vertically on the facet field instead of the group as initially done

{
  "$schema": "https://vega.github.io/schema/vega-lite/v6.json",
  "data": {
    "values": [
      {
        "group": "2025",
        "class": "A",
        "value": 23,
        "deltaX": 322,
        "class_color": "#041042"
      },
      {
        "group": "2025",
        "class": "B",
        "value": 21.5,
        "deltaX": 313,
        "class_color": "#7c31c0"
      },
      {
        "group": "2025",
        "class": "C",
        "value": 15,
        "deltaX": 275,
        "class_color": "#2652bc"
      }
    ]
  },
  "transform": [
    {
      "window": [
        {"field": "deltaX", "op": "max", "as": "deltaX_max"},
        {"field": "value", "op": "max", "as": "value_max"}
      ],
      "frame": [null, null]
    },
    {
      "calculate": "datum.deltaX / datum.deltaX_max * datum.value_max",
      "as": "value_published"
    },
    {"fold": ["value", "value_published"], "as": ["key", "value"]}
  ],
  /* Removed faceting */
  "encoding": {
    "x": {"field": "value", "type": "quantitative", "title": null},
    "y": {"field": "key" /* Was previously 'group', splitting here on the fold result for readability */, "type": "nominal", "title": null},
    "yOffset": {
      "field": "class",
      "type": "nominal",
      "sort": {"field": "y", "order": "ascending"}
    }
  },
  "layer": [
    {
      "mark": {"type": "bar", "tooltip": true},
      "encoding": {
        "color": {
          "field": "class",
          "type": "nominal",
          "scale": {"range": {"field": "class_color"}},
          "title": null
        }
      }
    },
    {
      "mark": {
        "type": "text",
        "align": "right",
        "xOffset": -4,
        "color": "white"
      },
      "encoding": {"text": {"field": "class", "type": "nominal"}}
    }
  ]
}

Checklist

• I checked for duplicate issues, but none seemed adequate.

Feel free to rename the issue with a better name than mine, as I'm not convinced the current one is very helpful.