Allow fallback color specification for incomplete domain-range mapping

[stefanv]

I have a scatter plot where the color of each point is determined by a certain field. I don't know all the values that the field can take, but I know that if that field has value "red", "green", or "blue", I want the points to appear in the corresponding color.

Currently, as per this StackOverflow answer, there is no way to provide discrete colors with a fallback.

[kanitw]

We should make the following work:

{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "description": "A simple bar chart with embedded data.",
  "data": {
    "values": [
      {"a": "A", "b": 28}, 
      {"a": "B", "b": 55}, 
      {"a": "C", "b": 43},
      {"a": "D", "b": 91}
    ]
  },
  "mark": "bar",
  "encoding": {
    "x": {"field": "a", "type": "ordinal", "axis": {"labelAngle": 0}},
    "y": {"field": "b", "type": "quantitative"},
    "color": {"field": "a", "type": "nominal", "scale": {
      "domain": {"unionWith": ["A", "C"]},
      "range": ["red", "green", "blue", "teal"]
    }, "sort": null}
  }
}

The tricky part is that:

{
  "name": "color",
  "type": "ordinal",
  "domain": {"fields": [{"data": "data_0", "field": "a"}, ["A", "B"]]},
  "range": ["red", "green", "blue", "teal"]
}

Besides, the other missing part is that you can't define custom mapping for a few colors and fill the rest with a predefined scheme

[stefanv]

@kanitw I was wondering if you had suggestions for a workaround to this issue? Sorry to bug.

[scizen9]

@kanitw is there any way to bump this up in priority? We are stuck until we can solve this or at least work around it. If there is anything we can do to help, please let us know. Thanks!