Should be

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "data": {
    "url": "data/cars.json"
  },
  "mark": "errorband",
  "encoding": {
    "y": {"field": "Miles_per_Gallon", "type": "quantitative"},
    "x": {"timeUnit": "year", "field": "Year"},
    "tooltip": {"field": "Miles_per_Gallon", "aggregate": "mean"}
  }
}

But doesn't work right now. Probably a problem in the normalizer.

The spec in #6184 (comment) works now and we should push it.

However, the tooltip doesn't work correctly because we remove the mean_Miles_per_Gallon from the compiled Vega spec. It's probably being removed in some optimization (https://github.com/vega/vega-lite/blob/main/src/compile/data/optimize.ts) where we recognize that the aggregate is the same as for center_Miles_per_Gallon but we don't correctly rewrite the output.