Population Pyramid Example

A population pyramid shows the distribution of age groups within a population. This example shows males and females across 150 years of U.S. census data. Drag the slider to see the U.S. population change over time (but watch out for missing data in 1890!).

Vega JSON Specification <>

{
  "$schema": "https://vega.github.io/schema/vega/v3.0.json",
  "height": 400,
  "padding": 5,

  "signals": [
    { "name": "chartWidth", "value": 300 },
    { "name": "chartPad", "value": 20 },
    { "name": "width", "update": "2 * chartWidth + chartPad" },
    { "name": "year", "value": 2000,
      "bind": {"input": "range", "min": 1850, "max": 2000, "step": 10} }
  ],

  "data": [
    {
      "name": "population",
      "url": "data/population.json"
    },
    {
      "name": "popYear",
      "source": "population",
      "transform": [
        {"type": "filter", "expr": "datum.year == year"}
      ]
    },
    {
      "name": "males",
      "source": "popYear",
      "transform": [
        {"type": "filter", "expr": "datum.sex == 1"}
      ]
    },
    {
      "name": "females",
      "source": "popYear",
      "transform": [
        {"type": "filter", "expr": "datum.sex == 2"}
      ]
    },
    {
      "name": "ageGroups",
      "source": "population",
      "transform": [
        { "type": "aggregate", "groupby": ["age"] }
      ]
    }
  ],

  "scales": [
    {
      "name": "y",
      "type": "band",
      "range": [{"signal": "height"}, 0],
      "round": true,
      "domain": {"data": "ageGroups", "field": "age"}
    },
    {
      "name": "c",
      "type": "ordinal",
      "domain": [1, 2],
      "range": ["#1f77b4", "#e377c2"]
    }
  ],

  "marks": [
    {
      "type": "text",
      "interactive": false,
      "from": {"data": "ageGroups"},
      "encode": {
        "enter": {
          "x": {"signal": "chartWidth + chartPad / 2"},
          "y": {"scale": "y", "field": "age", "band": 0.5},
          "text": {"field": "age"},
          "baseline": {"value": "middle"},
          "align": {"value": "center"},
          "fill": {"value": "#000"}
        }
      }
    },
    {
      "type": "group",

      "encode": {
        "update": {
          "x": {"value": 0},
          "height": {"signal": "height"}
        }
      },

      "scales": [
        {
          "name": "x",
          "type": "linear",
          "range": [{"signal": "chartWidth"}, 0],
          "nice": true, "zero": true,
          "domain": {"data": "population", "field": "people"}
        }
      ],

      "axes": [
        {"orient": "bottom", "scale": "x", "format": "s"}
      ],

      "marks": [
        {
          "type": "rect",
          "from": {"data": "females"},
          "encode": {
            "enter": {
              "x": {"scale": "x", "field": "people"},
              "x2": {"scale": "x", "value": 0},
              "y": {"scale": "y", "field": "age"},
              "height": {"scale": "y", "band": 1, "offset": -1},
              "fillOpacity": {"value": 0.6},
              "fill": {"scale": "c", "field": "sex"}
            }
          }
        }
      ]
    },
    {
      "type": "group",

      "encode": {
        "update": {
          "x": {"signal": "chartWidth + chartPad"},
          "height": {"signal": "height"}
        }
      },

      "scales": [
        {
          "name": "x",
          "type": "linear",
          "range": [0, {"signal": "chartWidth"}],
          "nice": true, "zero": true,
          "domain": {"data": "population", "field": "people"}
        }
      ],

      "axes": [
        {"orient": "bottom", "scale": "x", "format": "s"}
      ],

      "marks": [
        {
          "type": "rect",
          "from": {"data": "males"},
          "encode": {
            "enter": {
              "x": {"scale": "x", "field": "people"},
              "x2": {"scale": "x", "value": 0},
              "y": {"scale": "y", "field": "age"},
              "height": {"scale": "y", "band": 1, "offset": -1},
              "fillOpacity": {"value": 0.6},
              "fill": {"scale": "c", "field": "sex"}
            }
          }
        }
      ]
    }
  ]
}