One dot per airport in the US overlayed on geoshape

{
  "$schema": "https://vega.github.io/schema/vega-lite/v2.1.json",
  "width": 500,
  "height": 300,
  "layer": [
    {
      "data": {
        "url": "data/us-10m.json",
        "format": {
          "type": "topojson",
          "feature": "states"
        }
      },
      "projection": {
        "type": "albersUsa"
      },
      "mark": {
        "type": "geoshape",
        "fill": "lightgray",
        "stroke": "white"
      }
    },
    {
      "data": {
        "url": "data/airports.csv"
      },
      "projection": {
        "type": "albersUsa"
      },
      "mark": "circle",
      "encoding": {
        "longitude": {
          "field": "longitude",
          "type": "quantitative"
        },
        "latitude": {
          "field": "latitude",
          "type": "quantitative"
        },
        "size": {"value": 10},
        "color": {"value": "steelblue"}
      }
    }
  ]
}