Conditional expression to labelColor in header is not translated to Vega correctly

[joelostblom]

In this spec, I'm setting up conditional color encoding in the same manner for both axis labels and header labels:

  "encoding": {
    "column": {
      "field": "c",
      "header": {
        "labelExpr": "datum.value === \"One\" ? \"ONE\" : \"TWO\"",
        "labelColor": { "expr": "datum.value === \"One\" ? \"green\" : \"purple\""}
      },
      "type": "nominal"
    },
    "x": {
      "axis": {
        "labelExpr": "datum.value === \"A\" ? \"AA\" : \"BB\"",
        "labelColor": {"expr": "datum.value === \"A\" ? \"blue\" : \"red\""}
      },
      "field": "a",
      "type": "nominal"
    },
    "y": {"field": "b", "type": "quantitative"}
  }

However, it only works for the axis labels as can be seen in this screenshot:

Open the Chart in the Vega Editor

Looking at the Vega spec, it seems like VegaLite has translated the labelExpr and labelColor differently

      "title": {
        "text": {"signal": "parent[\"c\"] === \"One\" ? \"ONE\" : \"TWO\""},
        "style": "guide-label",
        "frame": "group",
        "color": {"signal": "datum.value === \"One\" ? \"green\" : \"purple\""},
        "offset": 10
      }

If I manually change datum.value to parent[\"c\"] for the "color" key, the header labels are colored as expected. If this is a simple case of duplicated some logic from labelExpr into labelColor, then I might be able to take a stab at a PR (although it might be a while until I get to it). Any pointers if that's the case and where this is handled in the code base?

A couple of other things I noted when working on this:

• datum.label and datum.value works for the axis config, but for the header config only datum.value works
• labelExpr takes an expression as a string but not as an expr object whereas labelColor does the opposite; it would be helpful for harmonize if possible.

Checklist

• I checked for duplicate issues.