Dominik's example with a reference line at the average

bars-with-average.json

{
  "description": "A simple bar chart with embedded data.",
  "data": {
    "values": [
      {"a": "A","b": 28},
      {"a": "B","b": 55},
      {"a": "C","b": 43},
      {"a": "D","b": 91},
      {"a": "E","b": 81},
      {"a": "F","b": 53},
      {"a": "G","b": 19},
      {"a": "H","b": 87},
      {"a": "I","b": 52}
    ]
  },
  "layers": [
    {
      "mark": "bar",
      "encoding": {
        "x": {"field": "a","type": "ordinal"},
        "y": {"field": "b","type": "quantitative"}
      }
    },
    {
      "mark": "rule",
      "config": {"mark": {"color": "#000"}},
      "encoding": {
        "y": {"type": "quantitative", "field": "b", "aggregate": "mean"}
      }
    }
  ]
}