going through the vega tutorial for re-creating the classic Mikey B viz on US airports

vega-flight-data.json

{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "width": 900,
  "height": 560,
  "padding": {"top": 0, "left": 0, "right": 0, "bottom": 0},
  "signals": [
    {
      "name": "hover",
      "value": null,
      "on": [
        {"events": "@airportmark:mouseover", "update": "datum"},
        {"events": "@airportmark:mouseout", "update": "null"}
      ]
    },
    {
      "name": "title",
      "value": "Flights 2008",
      "update": "hover ? hover.name + ' ('+hover.iata+', '+hover.traffic.flights+' flights)':'Flights 2008'"
    }
  ],
  "data": [
    {
      "name": "states",
      "url": "data/us-10m.json",
      "format": {"type": "topojson", "feature": "states"},
      "transform": [{"type": "geopath", "projection": "my_projection"}]
    },
    {
      "name": "traffic",
      "url": "data/flights-airport.csv",
      "format": {"type": "csv", "parse": "auto"},
      "transform": [
        {
          "type": "aggregate",
          "groupby": ["origin"],
          "fields": ["count"],
          "ops": ["sum"],
          "as": ["flights"]
        }
      ]
    },
    {
      "name": "traffic-raw",
      "url": "data/flights-airport.csv",
      "format": {"type": "csv", "parse": "auto"}
    },
    {
      "name": "airports-raw",
      "url": "data/airports.csv",
      "format": {"type": "csv", "parse": "auto"}
    },
    {
      "name": "airports",
      "url": "data/airports.csv",
      "format": {"type": "csv", "parse": "auto"},
      "transform": [
        {
          "type": "lookup",
          "from": "traffic",
          "key": "origin",
          "fields": ["iata"],
          "as": ["traffic"]
        },
        {"type": "filter", "expr": "datum.traffic != null"},
        {
          "type": "geopoint",
          "projection": "my_projection",
          "fields": ["longitude", "latitude"]
        },
        {"type": "filter", "expr": "datum.x != null && datum.y != null"},
        {
          "type": "collect",
          "sort": {"field": "traffic.flights", "order": "descending"}
        }
      ]
    },
    {
      "name": "routes",
      "url": "data/flights-airport.csv",
      "format": {"type": "csv", "parse": "auto"},
      "transform": [
        {"type": "filter", "expr": "hover && hover.iata==datum.origin"},
        {
          "type": "lookup",
          "from": "airports",
          "key": "iata",
          "fields": ["origin", "destination"],
          "as": ["source", "target"]
        },
        {"type": "filter", "expr": "datum.source && datum.target"},
        {"type": "linkpath", "shape": "curve"}
      ]
    }
  ],
  "scales": [
    {
      "name": "size",
      "type": "linear",
      "domain": {"data": "traffic", "field": "flights"},
      "range": [16, 2000]
    }
  ],
  "projections": [
    {
      "name": "my_projection",
      "type": "albersUsa",
      "scale": 1200,
      "translate": [{"signal": "width / 2"}, {"signal": "height / 2"}]
    }
  ],
  "marks": [
    {
      "type": "path",
      "from": {"data": "states"},
      "encode": {
        "enter": {"fill": {"value": "#dedede"}, "stroke": {"value": "white"}},
        "update": {"path": {"field": "path"}}
      }
    },
    {
      "type": "path",
      "interactive": false,
      "from": {"data": "routes"},
      "encode": {
        "enter": {
          "path": {"field": "path"},
          "stroke": {"value": "black"},
          "strokeOpacity": {"value": 0.5}
        }
      }
    },
    {
      "type": "symbol",
      "name": "airportmark",
      "from": {"data": "airports"},
      "encode": {
        "enter": {
          "shape": {"value": "square"},
          "size": {"scale": "size", "field": "traffic.flights"},
          "fill": {"value": "teal"},
          "fillOpacity": {"value": 0.8},
          "stroke": {"value": "white"},
          "strokeWidth": {"value": 1.5}
        },
        "update": {"x": {"field": "x"}, "y": {"field": "y"}}
      }
    },
    {
      "type": "text",
      "interactive": false,
      "encode": {
        "enter": {
          "x": {"signal": "width", "offset": -5},
          "y": {"value": 25},
          "fill": {"value": "black"},
          "align": {"value": "right"}
        },
        "update": {"text": {"signal": "title"}}
      }
    }
  ]
}