density2.vl

{
    "transform": [
        {
            "as": [
                "measurement",
                "value"
            ],
            "fold": [
                "petalWidth",
                "petalLength",
                "sepalWidth",
                "sepalLength"
            ]
        },
        {
            "groupby": [
                "measurement"
            ],
            "extent": [
                0,
                8
            ],
            "steps": 200,
            "bandwidth": 0.3,
            "density": "value"
        }
    ],
    "height": 100,
    "mark": {
        "opacity": 0.5,
        "type": "area"
    },
    "data": {
        "url": "https://vega.github.io/vega-lite/data/iris.json"
    },
    "width": 400,
    "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
    "encoding": {
        "color": {
            "field": "measurement",
            "type": "nominal"
        },
        "x": {
            "field": "value",
            "title": "width/length (cm)",
            "type": "quantitative"
        },
        "y": {
            "field": "density",
            "type": "quantitative"
        }
    }
}