Histogram with a Global Mean Overlay

{
  "$schema": "https://vega.github.io/schema/vega-lite/v2.json",
  "data": {"url": "data/movies.json"},
  "layer": [{
    "mark": "bar",
    "encoding": {
      "x": {
        "bin": true,
        "field": "IMDB_Rating",
        "type": "quantitative"
      },
      "y": {
        "aggregate": "count",
        "type": "quantitative"
      }
    }
  },{
    "mark": "rule",
    "encoding": {
      "x": {
        "aggregate": "mean",
        "field": "IMDB_Rating",
        "type": "quantitative"
      },
      "color": {"value": "red"},
      "size": {"value": 5}
    }
  }]
}