Categorical "count" field on color doesn't work for line and area

[yhoonkim]

Bug Description

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "data": {
    "values": [
      {"category":"A", "count": 1, "value":0.1},
      {"category":"A", "count": 2, "value":0.6},
      {"category":"A", "count": 3, "value":0.9},
      {"category":"B", "count": 1, "value":0.7},
      {"category":"B", "count": 2, "value":0.2},
      {"category":"B", "count": 3, "value":1.1},
      {"category":"C", "count": 1, "value":0.6},
      {"category":"C", "count": 2, "value":0.1},
      {"category":"C", "count": 3, "value":0.2}
    ]
  },
  "mark": "line",
  "encoding": {
    "x": {"field": "category"},
    "y": {"field": "value", "type": "quantitative"},
    "color": {"field": "count"}
  }
}

Note that if we change "count" -> "_count", it works well:

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "data": {
    "values": [
      {"category":"A", "_count": 1, "value":0.1},
      {"category":"A", "_count": 2, "value":0.6},
      {"category":"A", "_count": 3, "value":0.9},
      {"category":"B", "_count": 1, "value":0.7},
      {"category":"B", "_count": 2, "value":0.2},
      {"category":"B", "_count": 3, "value":1.1},
      {"category":"C", "_count": 1, "value":0.6},
      {"category":"C", "_count": 2, "value":0.1},
      {"category":"C", "_count": 3, "value":0.2}
    ]
  },
  "mark": "line",
  "encoding": {
    "x": {"field": "category"},
    "y": {"field": "value", "type": "quantitative"},
    "color": {"field": "_count"}
  }
}

It seems like related to Vega group mark’s “from.facet”; it creates subfacets with “count” which interferes the group_by “count” value. E.g.,:

Checklist

• I checked for duplicate issues.

[domoritz]

Yeah, we should use some clear pattern for internal fields. Maybe _{} so here _count. Can you send a pull request to fix this?

[yhoonkim]

@domoritz Sorry, I have no capacity to fix this now. If you have any hint for where I/other people should start to look at, it will be super helpful :)

[kanitw]

This is probably coming from facet operator in Vega.