Line Chart with Confidence Interval Band

{
  "$schema": "https://vega.github.io/schema/vega-lite/v2.json",
  "data": {"url": "data/cars.json"},
  "layer": [
    {
      "mark": "area",
      "encoding": {
        "x": {
          "field": "Year",
          "type": "temporal",
          "timeUnit": "year"
        },
        "y": {
          "aggregate": "ci0",
          "field": "Miles_per_Gallon",
          "type": "quantitative",
          "axis": {"title": "Mean of Miles per Gallon (95% CIs)"}
        },
        "y2": {
          "aggregate": "ci1",
          "field": "Miles_per_Gallon",
          "type": "quantitative"
        },
        "opacity": {"value": 0.3}
      }
    },
    {
      "mark": "line",
      "encoding": {
        "x": {
          "field": "Year",
          "type": "temporal",
          "timeUnit": "year"
        },
        "y": {
          "aggregate": "mean",
          "field": "Miles_per_Gallon",
          "type": "quantitative"
        }
      }
    }
  ]
}