highlightvalue.vl

{
    "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
    "layer": [
        {
            "data": {
                "url": "https://vega.github.io/vega-lite/data/stocks.csv"
            },
            "encoding": {
                "color": {
                    "field": "symbol",
                    "type": "nominal"
                },
                "x": {
                    "field": "date",
                    "type": "temporal"
                },
                "y": {
                    "field": "price",
                    "type": "quantitative"
                }
            },
            "mark": "line"
        },
        {
            "data": {
                "values": {}
            },
            "encoding": {
                "y": {
                    "datum": 300
                }
            },
            "mark": {
                "size": 2,
                "strokeDash": [
                    2,
                    2
                ],
                "type": "rule"
            }
        }
    ]
}