gradient2.vl

{
    "mark": {
        "color": {
            "x2": 1,
            "y1": 1,
            "stops": [
                {
                    "color": "red",
                    "offset": 0
                },
                {
                    "color": "blue",
                    "offset": 1
                }
            ],
            "gradient": "linear",
            "y2": 1,
            "x1": 0
        },
        "type": "bar"
    },
    "data": {
        "values": [
            {
                "value": 10,
                "cat": "a"
            },
            {
                "value": 5,
                "cat": "b"
            },
            {
                "value": 20,
                "cat": "c"
            },
            {
                "value": 8,
                "cat": "d"
            }
        ]
    },
    "width": 200,
    "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
    "encoding": {
        "x": {
            "field": "cat",
            "type": "nominal"
        },
        "y": {
            "field": "value",
            "type": "quantitative"
        }
    }
}