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{  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",  "description": "Using `labelExpr` to show only initial letters of month names.",  "data": {"url": "data/seattle-weather.csv"},  "mark": "bar",  "encoding": {    "x": {      "timeUnit": "month",      "field": "date",      "axis": {        "labelAlign": "left",        "labelExpr": "datum.label[0]"      }    },    "y": {"aggregate": "mean", "field": "precipitation"}  }}