flatten1.vl

{
    "transform": [
        {
            "as": [
                "quant",
                "cat"
            ],
            "flatten": [
                "foo",
                "bar"
            ]
        }
    ],
    "mark": "circle",
    "data": {
        "values": [
            {
                "foo": [
                    1,
                    2
                ],
                "key": "alpha",
                "bar": [
                    "A",
                    "B"
                ]
            },
            {
                "foo": [
                    3,
                    4,
                    5
                ],
                "key": "beta",
                "bar": [
                    "C",
                    "D"
                ]
            }
        ]
    },
    "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
    "encoding": {
        "color": {
            "field": "key",
            "type": "nominal"
        },
        "x": {
            "field": "quant",
            "type": "quantitative"
        },
        "y": {
            "field": "cat",
            "type": "nominal"
        }
    }
}