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    {
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      "bind": {"input": "select", "options": [1, 5, 10, 20]}
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  "title": {
    "text": "Density of U.S. Airports, 2008",
    "offset": -15
  },

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      "url": "data/us-10m.json",
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          "type": "geopath",
          "projection": "projection"
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    },
    {
      "name": "airports",
      "url": "data/airports.csv",
      "format": {"type": "csv","parse": "auto"
      },
      "transform": [
        {
          "type": "geopoint",
          "projection": "projection",
          "fields": ["longitude", "latitude"]
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          "expr": "datum.x != null && datum.y != null"
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      "type": "path",
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      "from": {"data": "states"},
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          "fill": {"value": "#dedede"},
          "stroke": {"value": "white"}
        },
        "update": {
          "path": {"field": "path"}
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      }
    },
    {
      "type": "symbol",
      "clip": true,
      "from": {"data": "airports"},
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        },
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          "x": {"field": "x"},
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      }
    },
    {
      "type": "group",
      "clip": true,
      "data": [
        {
          "name": "contours",
          "source": "airports",
          "transform": [
            {
              "type": "contour",
              "x": "x",
              "y": "y",
              "size": [{"signal": "width"}, {"signal": "height"}],
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