Using the lookup transform to combine data

{
  "$schema": "https://vega.github.io/schema/vega-lite/v2.json",
  "data": {"url": "data/lookup_groups.csv"},
  "transform": [{
    "lookup": "person",
    "from": {
      "data": {"url": "data/lookup_people.csv"},
      "key": "name",
      "fields": ["age", "height"]
    }
  }],
  "mark": "bar",
  "encoding": {
    "x": {"field": "group", "type": "ordinal"},
    "y": {"field": "age", "aggregate": "mean", "type": "quantitative"}
  }
}