Vega-Lite: Negative values in the size channel

spec.json

{
    "$schema": "https://vega.github.io/schema/vega-lite/v4.8.1.json",
    "title": "Negative values in the size channel",
    "data": {
        "values": [-4, -3, -2, -1, 0, 1, 2, 3, 4]
    },
    "transform": [
        {"calculate": "abs(datum.data)", "as": "abs"}
    ],
    "repeat": ["data", "abs"],
    "spec": {
        "mark": "circle",
        "encoding": {
            "x": {
                "type": "ordinal",
                "field": "data"
            },
            "size": {
                "type": "quantitative",
                "field": {"repeat": "repeat"}
            },
            "color": {
                "condition": {
                    "test": "datum['data'] < 0", 
                    "value": "red"
                },
                "value": "blue"
            }
        }
    },
    "resolve": {
        "scale": {"size": "independent"}
    }
}