Deneb (Vega-lite) - Access Other Columns in Axis Label Expression

I am attempting to use expr to format/style my Yaxis labelFont, but it does not seem to take the expression. I have a helper column called YLabelFontFamily that already has values for reach row.

According to the documentation, labelFont accepts ExprRef, when I specify "datum.YLabelFontFamily", the font is not applied. What am I doing wrong?

See screenshot below.

The pbix file can be found at https://github.com/triCNguyen5/Test/blob/main/YaxisLabelTest.pbix

The axis doesn't have access to the datapoint. It can only see the value which is accessed via datum.label or datum.value (both return the same thing). e.g.

{
  "$schema": "https://vega.github.io/schema/vega-lite/v6.json",
  "description": "A simple bar chart with embedded data.",
  "data": {
    "values": [
      {"a": "A", "b": 28},
      {"a": "B", "b": 55},
      {"a": "C", "b": 43}
    ]
  },
  "mark": "bar",
  "encoding": {
    "y": {
      "field": "a",
      "type": "nominal",
      "axis": {"labelFont": {"expr": "datum.label=='A'?'Arial Black':'Arial'"}}
    },
    "x": {"field": "b", "type": "quantitative"}
  }
}

You can achieve your end goal as follows:

{
  "$schema": "https://vega.github.io/schema/vega-lite/v6.json",
  "description": "A simple bar chart with embedded data.",
  "data": {
    "values": [
      {"a": "A", "b": 28, "c":"Arial"},
      {"a": "B", "b": 55, "c":"Arial Black"},
      {"a": "C", "b": 43, "c":"Arial"}
    ]
  }, "transform": [{"calculate": "datum.a+'|'+datum.c" ,"as": "d"}],
  "mark": "bar",
  "encoding": {
    "y": {
      "field": "d",
      "type": "nominal",
      "axis": {"labelFont": {"expr": "split(datum.label,'|')[1]"}, "labelExpr": "split(datum.label,'|')[0]", "title":"a"}
    },
    "x": {"field": "b", "type": "quantitative"}
  }
}