boxplot3.vl

{
    "mark": {
        "median": {
            "strokeWidth": 0,
            "size": 18,
            "fill": "black"
        },
        "box": {
            "color": "firebrick"
        },
        "rule": {
            "strokeWidth": 0.4
        },
        "extent": 0.5,
        "outliers": {
            "strokeWidth": 0.3,
            "color": "black",
            "size": 10
        },
        "type": "boxplot",
        "ticks": {
            "size": 8
        }
    },
    "data": {
        "url": "https://vega.github.io/vega-lite/data/population.json"
    },
    "$schema": "https://vega.github.io/schema/vega-lite/v3.json",
    "encoding": {
        "x": {
            "field": "age",
            "type": "ordinal"
        },
        "y": {
            "field": "people",
            "type": "quantitative",
            "axis": {
                "title": "Population"
            }
        }
    }
}