Mean overlay over precipitation chart

{
  "$schema": "https://vega.github.io/schema/vega-lite/v2.json",
  "data": {"url": "data/seattle-weather.csv"},
  "layer": [
    {
      "mark": "bar",
      "encoding": {
        "x": {
          "timeUnit": "month",
          "field": "date",
          "type": "ordinal"

        },
        "y": {
          "aggregate": "mean",
          "field": "precipitation",
          "type": "quantitative"
        }
      }
    },
    {
      "mark": "rule",
      "encoding": {
        "y": {
          "aggregate": "mean",
          "field": "precipitation",
          "type": "quantitative"
        },
        "color": {"value": "red"},
        "size": {"value": 3}
      }
    }
  ]
}