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  "width": 900,
  "height": 560,
  "padding": 0,
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      "bind": {"input": "range", "min": -1, "max": 100, "step": 1}
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    },
    {
      "name": "levels", "value": 10,
      "bind": {"input": "select", "options": [1, 5, 10, 20]}
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  ],

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

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      "url": "data/us-10m.json",
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      "transform": [
        {
          "type": "geopath",
          "projection": "projection"
        }
      ]
    },
    {
      "name": "airports",
      "url": "data/airports.csv",
      "format": {"type": "csv","parse": "auto"
      },
      "transform": [
        {
          "type": "geopoint",
          "projection": "projection",
          "fields": ["longitude", "latitude"]
        },
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      ]
    },
    {
      "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
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  "scales": [
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      "name": "color",
      "type": "linear",
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  "marks": [
    {
      "type": "path",
      "clip": true,
      "from": {"data": "states"},
      "encode": {
        "enter": {
          "fill": {"value": "#dedede"},
          "stroke": {"value": "white"}
        },
        "update": {
          "path": {"field": "path"}
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      }
    },
    {
      "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}
        },
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    },
    {
      "type": "shape",
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      "from": {"data": "contours"},
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          "strokeWidth": {"value": 0}
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