layered2.vl

{
    "transform": [
        {
            "filter": "datum.symbol === 'GOOG'"
        }
    ],
    "data": {
        "url": "https://vega.github.io/vega-lite/data/stocks.csv"
    },
    "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
    "layer": [
        {
            "mark": {
                "opacity": 0.3,
                "type": "point"
            },
            "encoding": {
                "x": {
                    "field": "date",
                    "timeUnit": "year",
                    "type": "temporal"
                },
                "y": {
                    "field": "price",
                    "type": "quantitative"
                }
            }
        },
        {
            "mark": "line",
            "encoding": {
                "x": {
                    "field": "date",
                    "timeUnit": "year",
                    "type": "temporal"
                },
                "y": {
                    "field": "price",
                    "aggregate": "mean",
                    "type": "quantitative"
                }
            }
        }
    ],
    "description": "Plot showing average data with raw values in the background."
}