initialletter.vl

{
    "height": 300,
    "mark": {
        "cornerRadius": 10,
        "strokeWidth": 4,
        "color": {
            "stops": [
                {
                    "color": "orange",
                    "offset": 0
                },
                {
                    "color": "cyan",
                    "offset": 1
                }
            ],
            "gradient": "linear"
        },
        "blend": "difference",
        "stroke": "yellow",
        "tooltip": {
            "content": "data"
        },
        "type": "bar"
    },
    "data": {
        "url": "https://vega.github.io/vega-lite/data/seattle-weather.csv"
    },
    "width": 400,
    "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
    "encoding": {
        "x": {
            "field": "date",
            "timeUnit": "month",
            "type": "temporal",
            "axis": {
                "labelExpr": "datum.label[0]",
                "labelAlign": "left"
            }
        },
        "y": {
            "field": "precipitation",
            "aggregate": "mean",
            "type": "quantitative"
        }
    },
    "description": "Using `labelExpr` to show only initial letters of month names."
}