{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "width": 900,
  "height": 560,
  "padding": 0,
  "autosize": "none",

  "signals": [
    {
      "name": "bandwidth", "value": 20,
      "bind": {"input": "range", "min": -1, "max": 100, "step": 1}
    },
    {
      "name": "cellSize", "value": 4,
      "bind": {"input": "select", "options": [1, 2, 4, 8, 16, 32]}
    },
    {
      "name": "levels", "value": 10,
      "bind": {"input": "select", "options": [1, 5, 10, 20]}
    }
  ],

  "title": {
    "text": "Density of U.S. Airports, 2008",
    "offset": -20
  },

  "data": [
    {
      "name": "states",
      "url": "data/us-10m.json",
      "format": {"type": "topojson", "feature": "states"},
      "transform": [
        {
          "type": "geopath",
          "projection": "projection"
        }
      ]
    },
    {
      "name": "airports",
      "url": "data/airports.csv",
      "format": {"type": "csv","parse": "auto"
      },
      "transform": [
        {
          "type": "geopoint",
          "projection": "projection",
          "fields": ["longitude", "latitude"]
        },
        {
          "type": "filter",
          "expr": "datum.x != null && datum.y != null"
        }
      ]
    },
    {
      "name": "contours",
      "source": "airports",
      "transform": [
        {
          "type": "kde2d",
          "x": "x",
          "y": "y",
          "size": [{"signal": "width"}, {"signal": "height"}],
          "bandwidth": {"signal": "[bandwidth, bandwidth]"},
          "cellSize": {"signal": "cellSize"}
        },
        {
          "type": "isocontour",
          "field": "grid",
          "levels": {"signal": "levels"},
          "as": null
        }
      ]
    }
  ],

  "projections": [
    {
      "name": "projection",
      "type": "albers",
      "scale": 1150,
      "translate": [{"signal": "width / 2"}, {"signal": "height / 2"}]
    }
  ],

  "scales": [
    {
      "name": "color",
      "type": "linear",
      "domain": {"data": "contours", "field": "value"},
      "range": {"scheme": "viridis"}
    }
  ],

  "marks": [
    {
      "type": "path",
      "clip": true,
      "from": {"data": "states"},
      "encode": {
        "enter": {
          "fill": {"value": "#dedede"},
          "stroke": {"value": "white"}
        },
        "update": {
          "path": {"field": "path"}
        }
      }
    },
    {
      "type": "symbol",
      "clip": true,
      "from": {"data": "airports"},
      "encode": {
        "enter": {
          "size": {"value": 10},
          "fill": {"value": "steelblue"},
          "fillOpacity": {"value": 0.8},
          "stroke": {"value": "white"},
          "strokeWidth": {"value": 0.5}
        },
        "update": {
          "x": {"field": "x"},
          "y": {"field": "y"}
        }
      }
    },
    {
      "type": "shape",
      "clip": true,
      "from": {"data": "contours"},
      "encode": {
        "enter": {
          "stroke": {"value": "#000"},
          "fill": {"scale": "color", "field": "value"},
          "fillOpacity": {"value": 0.4}
        },
        "update": {
          "strokeWidth": {"value": 0}
        },
        "hover": {
          "strokeWidth": {"value": 1.5}
        }
      },
      "transform": [
        { "type": "geoshape", "field": "datum" }
      ]
    }
  ]
}
