Support for strokeWidth scale in legend

When trying to encode a field as “strokeWidth” in the editor with vega-lite 3.0.0-rc10, the following error is raised: “Error: Missing valid scale for legend.” Setting “strokeWidth” to a constant value does seem to work.

{
  "$schema": "https://vega.github.io/schema/vega-lite/v3.json",
  "description": "A simple bar chart with embedded data.",
  "data": {
    "values": [
      {"a": "A","b": 28}, {"a": "B","b": 55}, {"a": "C","b": 43}
    ]
  },
  "mark": "square",
  "encoding": {
    "x": {"field": "a", "type": "ordinal"},
    "y": {"field": "b", "type": "quantitative"},
    "strokeWidth": {"field": "b", "type": "quantitative"}
  }
}

[kanitw]

I think supporting legend for fill/strokeOpacity is lower priority than for strokeWidth.

[kanitw]

https://github.com/vega/vega-lite/pull/4454 disables legend for fill/strokeOpacity and strokeWidth in Vega-Lite. We can enable them again once we support any of these in Vega.