Dorling Cartogram Example

A Dorling cartogram is a thematic map that uses sized circles to represent a quantity of interest per geographic region. This example visualizes the ratio of obese adults (BMI >= 30) by U.S. state in 2008. A redundant encoding uses both circle area and fill color to convey the obesity rate. Vega’s force transform and geoCentroid expression function are used to compute the layout.

Vega JSON Specification <>

{
  "$schema": "https://vega.github.io/schema/vega/v3.0.json",
  "width": 900,
  "height": 520,
  "autosize": "none",

  "config": {
    "legend": {
      "gradientWidth": 120,
      "gradientHeight": 10
    }
  },

  "data": [
    {
      "name": "states",
      "url": "data/us-10m.json",
      "format": {"type": "topojson", "feature": "states"}
    },
    {
      "name": "obesity",
      "url": "data/obesity.json",
      "transform": [
        {
          "type": "lookup",
          "from": "states", "key": "id",
          "fields": ["id"], "as": ["geo"]
        },
        {
          "type": "filter",
          "expr": "datum.geo"
        },
        {
          "type": "formula", "as": "centroid",
          "expr": "geoCentroid('projection', datum.geo)"
        }
      ]
    }
  ],

  "projections": [
    {
      "name": "projection",
      "type": "albersUsa",
      "scale": 1100,
      "translate": [{"signal": "width / 2"}, {"signal": "height / 2"}]
    }
  ],

  "scales": [
    {
      "name": "size",
      "domain": {"data": "obesity", "field": "rate"},
      "range": [0, 5000]
    },
    {
      "name": "color",
      "type": "sequential",
      "nice": true,
      "domain": {"data": "obesity", "field": "rate"},
      "range": "ramp"
    }
  ],

  "legends": [
    {
      "title": "% of Obese Adults",
      "orient": "bottom-right",
      "type": "gradient",
      "fill": "color",
      "format": ".1%"
    }
  ],

  "marks": [
    {
      "name": "circles",
      "type": "symbol",
      "from": {"data": "obesity"},
      "encode": {
        "enter": {
          "size": {"scale": "size", "field": "rate"},
          "fill": {"scale": "color", "field": "rate"},
          "stroke": {"value": "white"},
          "strokeWidth": {"value": 1.5},
          "x": {"field": "centroid[0]"},
          "y": {"field": "centroid[1]"},
          "tooltip": {"signal": "'Obesity Rate: ' + format(datum.rate, '.1%')"}
        }
      },
      "transform": [
        {
          "type": "force",
          "static": true,
          "forces": [
            {"force": "collide", "radius": {"expr": "1 + sqrt(datum.size) / 2"}},
            {"force": "x", "x": "datum.centroid[0]"},
            {"force": "y", "y": "datum.centroid[1]"}
          ]
        }
      ]
    },
    {
      "type": "text",
      "interactive": false,
      "from": {"data": "circles"},
      "encode": {
        "enter": {
          "align": {"value": "center"},
          "baseline": {"value": "middle"},
          "fontSize": {"value": 13},
          "fontWeight": {"value": "bold"},
          "text": {"field": "datum.state"}
        },
        "update": {
          "x": {"field": "x"},
          "y": {"field": "y"}
        }
      }
    }
  ]
}