Deneb - Vega Lite Color Gradient Per Category

DW868990, oh wow, apologies. It was so similar to an example on the vega-lite page, that I missed what you were actually after. Let me know if this is better. Happy to make additional tweaks if necessary. I took out the text in the boxes to match what you have. Can add it back in though if you like.

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "data": {"url": "data/cars.json"},
  "facet": {"row": {"field": "Origin", "title": null}},
  "title": {
    "text": "Mean Horsepower per Cylinder per Origin",
    "orient": "top",
    "anchor": "middle"
  },
  "spec": {
    "encoding": {
      "y": {"field": "Origin", "type": "ordinal", "axis": null},
      "x": {"field": "Cylinders", "type": "ordinal"}
    },
    "layer": [
      {
        "mark": "rect",
        "encoding": {
          "color": {
            "field": "Horsepower",
            "aggregate": "average",
            "type": "quantitative",
            "legend": {
              "direction": "horizontal",
              "orient": "right",
              "gradientLength": 100,
              "title": null
            }
          }
        }
      }
    ]
  },
  "resolve": {"scale": {"y": "independent", "color": "independent"}},
  "config": {"axis": {"grid": true, "tickBand": "extent"}}
}