Layering mean values over raw values

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Vega-Lite JSON Specification

{
  "$schema": "https://vega.github.io/schema/vega-lite/v3.json",
  "description": "Plot showing average data with raw values in the background.",
  "data": {"url": "data/stocks.csv"},
  "transform": [{"filter": "datum.symbol==='GOOG'"}],
  "layer": [{
    "mark": {"type": "point", "opacity": 0.3},
    "encoding": {
      "x": {"timeUnit":"year","field": "date", "type": "temporal"},
      "y": {"field": "price", "type": "quantitative"}
    }
  }, {
    "mark": "line",
    "encoding": {
      "x": {"timeUnit":"year", "field": "date", "type": "temporal"},
      "y": {"aggregate": "mean", "field": "price", "type": "quantitative"}
    }
  }]
}