One dot per zipcode in the U.S

{
  "$schema": "https://vega.github.io/schema/vega-lite/v2.1.json",
  "width": 500,
  "height": 300,
  "data": {
    "url": "data/zipcodes.csv"
  },
  "transform": [{"calculate": "substring(datum.zip_code, 0, 1)", "as": "digit"}],
  "projection": {
    "type": "albersUsa"
  },
  "mark": "circle",
  "encoding": {
    "longitude": {
      "field": "longitude",
      "type": "quantitative"
    },
    "latitude": {
      "field": "latitude",
      "type": "quantitative"
    },
    "size": {"value": 1},
    "color": {"field": "digit", "type": "nominal"}
  }
}