This entry is part of the Chart Index, the reference library for the Chart Design Field Guide.

A cartogram redraws a map so that geographic area represents a quantitative value rather than land. A populous state becomes larger; a sparsely populated state becomes smaller. The geography is deliberately distorted so the visual weight of each region follows the subject of the analysis.

That trade is the form's purpose. A conventional map answers where and how much land? A cartogram answers where and how much of the thing being measured?

What it is

Every region begins with a geographic location and a value. The layout then scales each region's area to the value while preserving enough neighbourhood and relative position for the map to remain recognisable.

Regional demand indexCircle area represents the measure; position preserves the broad geography

The example is a Dorling cartogram: regions are replaced with circles positioned near their geographic centres. Circle area carries the value. Collision handling separates overlapping regions while attraction to the original positions keeps the national shape legible.

When to use it

  • Geographic area would give large, lightly populated regions too much visual weight.
  • The reader needs to compare regional totals rather than rates.
  • Broad location and neighbourhood still matter, but exact boundaries do not.
  • The distortion itself helps explain an imbalance, concentration, or mismatch.
  • The audience can be given enough labels and context to recognise the transformed geography.

When not to use it

  • Exact boundaries, routes, distances, or locations are part of the question.
  • The geography is unfamiliar and distortion would make regions impossible to identify.
  • Precise value comparison is required. A sorted bar chart will be faster and more accurate.
  • There are too many small regions to label or distinguish at the intended size.
  • The map is being used only because the data contains place names. Geography must contribute to the reading task.

Design principles

Choose the cartogram family deliberately

Contiguous cartograms distort region shapes while preserving shared boundaries. They retain adjacency but can produce unfamiliar outlines.

Non-contiguous cartograms scale regions independently around fixed centres. Shapes remain recognisable, but gaps open between neighbours.

Dorling cartograms replace regions with circles. They sacrifice boundaries and shape for clean area comparison, reliable packing, and compact labels.

The Dorling form is often the strongest choice for a business audience because its contract is visible: circle size is data, circle position is geography.

Encode value with area, not radius

If one region represents four times the value of another, its mark needs four times the area, not four times the radius. Scaling radius directly exaggerates differences because circle area grows with the square of the radius.

Preserve the geographic skeleton

Keep regions near their true positions and preserve important neighbourhoods where possible. A cartogram should still read as a transformed map. Once the marks are freely rearranged by rank, the result is a bubble chart.

Provide orientation

Use direct region labels, a familiar national outline, or a small conventional inset map. Distortion removes geographic cues, so the design needs to restore the minimum context required for recognition.

Use colour for a second job only

Area already encodes the primary measure. A single restrained hue is often sufficient. Add colour only for a meaningful second variable, such as region type, change direction, or uncertainty, and provide a clear legend.

Make small regions selectable

Small values create small marks. Give them generous interaction targets, concise tooltips, and labels that remain legible without covering neighbouring regions. Do not let the smallest places disappear merely because their measured value is low.

State what has been distorted

Name the measure encoded by area, its unit, the period, and whether the values are totals or rates. Readers should never have to infer why the map no longer resembles the geography they know.

Anatomy

A cartogram combines five elements:

  • Geographic anchor — the original centre or location of each region.
  • Area-scaled mark — a transformed polygon or circle whose area represents the value.
  • Displacement — movement required to prevent overlap while retaining broad position.
  • Region identifier — a label or interaction target that makes the transformed geography readable.
  • Area legend — reference sizes when readers need more than relative comparison.

In a Dorling cartogram, the layout balances two competing forces: collision pushes circles apart, while geographic attraction pulls each circle towards its original location. The result is not a conventional map and should not be read as one; position is approximate, while area is analytical.

  • Choropleth — preserves geographic shape and encodes a normalised measure with colour.
  • Value-by-alpha map — keeps boundaries but reduces the visual weight of regions with little supporting data.
  • Hexbin and H3 map — replaces administrative regions with uniform spatial cells for point density.
  • Tilegram — allocates equal, countable tiles to regions while preserving broad geography.
  • Bubble chart — compares area-scaled circles without a geographic constraint.
  • Geographic base map — preserves location and boundaries when geometry itself matters.

Reading list

  • Dorling, D. (1996). Area Cartograms: Their Use and Creation.
  • Gastner, M. T. and Newman, M. E. J. (2004). Diffusion-based method for producing density-equalizing maps.
  • Nusrat, S. and Kobourov, S. (2016). The State of the Art in Cartograms.