Error Bars showing Confidence Interval

{
  "$schema": "https://vega.github.io/schema/vega-lite/v2.json",
  "data": {"url": "data/barley.json"},
  "layer": [{
    "mark": {"type": "point", "filled": true},
    "encoding": {
      "x": {
        "aggregate": "mean", "field": "yield", "type": "quantitative",
        "scale": {"zero": false},
        "axis": {"title": "Barley Yield"}
      },
      "y": {
        "field": "variety", "type": "ordinal"
      },
      "color": {"value": "black"}
    }
  }, {
    "mark": "rule",
    "encoding": {
      "x": {
        "aggregate": "ci0", "field": "yield", "type": "quantitative"
      },
      "x2": {
        "aggregate": "ci1", "field": "yield", "type": "quantitative"
      },
      "y": {
        "field": "variety", "type": "ordinal"
      }
    }
  }]
}