fillopacity.vl

{
    "mark": "bar",
    "data": {
        "values": [
            {
                "a": "A",
                "b": 10
            },
            {
                "a": "B",
                "b": 20
            }
        ]
    },
    "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
    "encoding": {
        "x": {
            "field": "a",
            "type": "ordinal",
            "axis": {
                "labelAngle": 0
            }
        },
        "fillOpacity": {
            "field": "b",
            "type": "quantitative"
        },
        "y": {
            "field": "b",
            "type": "quantitative"
        }
    }
}