When a map is the right choice, when a bar chart or a table reads better, and how to draw a map that does not mislead. The same rules hold in every chart library. For picking between chart types in general, see Choosing a chart.
What it is for
A map shows a value where it happens. The shapes come from real geography, or from any plan with coordinates, such as a building floor.
A choropleth shades each region by its value, such as the unemployment rate of each county.
A bubble map puts a circle on each place, sized by an amount such as population.
A route map draws a path across the map, such as a voyage.
When to use
Location is part of the question: which areas are high, and whether the high areas sit together.
The reader knows the geography and looks for their own region, or for a pattern such as a north to south gradient.
You show where events happened or which way a route went.
Use something else when
Location does not matter to the question. Use a sorted bar chart of the regions.
The reader ranks regions or compares them by value. Use a sorted bar chart, or add one beside the map.
The regions differ a lot in size and the small ones matter, such as city states or small counties. Use a bar chart, a table, or a tile map with one equal square per region.
There are only a few regions. Use a bar chart or a table.
You show a change over time for each region. Use a line chart per region, or small multiples of the map for a few dates.
The reader needs exact values. Use a table, sortable by region and by value.
How it reads
A choropleth encodes the value as color, which people read less accurately than length or position. Readers see bands of light and dark, not exact numbers. Large regions also draw the eye, so a big, empty region looks more important than a small, crowded one with the same value.
A bubble map encodes the value as the area of a circle. Area is also hard to compare, so a bubble map shows where the big values are, not by how much they differ.
Do it well
Shade a choropleth by a rate, not a total: per person, per square kilometer, or a share. Totals mostly show where people live.
Show totals, such as population, as circles on a bubble map, sized by area, not by radius.
Use a sequential palette for values that only grow, and a diverging palette only around a meaningful center, such as zero change.
Group values into five to seven bands, and show the bands in a legend with their ranges.
Give regions with no data their own gray, and say so in the legend. A missing value is not zero.
Pick an equal-area projection, such as Equal Earth, for a world choropleth, so each country keeps its true relative size.
Keep borders thin and in the page color, so they separate regions without hiding small ones.
Pitfalls
A map of totals. The most populous regions win every map. Divide by population or area first.
Mercator for a world map. It inflates land near the poles, so Greenland looks as large as Africa. Use it only for street maps and navigation.
Small regions that vanish. A small country or a city county is a few pixels wide, and the reader cannot see or hover it. Add a table, an inset map, or zoom.
Too many color bands, or a rainbow palette. Readers cannot match the shades to the legend.
A legend that changes with a filter. If one region leaves the data, every other color must keep its meaning. Fix the color domain.
Bubbles sized by radius. A value twice as large looks four times as large.
Variants
Choropleth with thresholds: fixed bands with a meaning of their own, such as under 20%.
Choropleth with equal intervals: the value range split into bands of the same width.
Choropleth with quantiles: the same number of regions in each band, for skewed data.
Bubble map: a circle per place, sized by an amount.
Route map: a line across the map, for a path or a trip.
Globe: an orthographic view of one side of the earth, for context more than comparison.
Plan: shapes in plain coordinates, such as a floor plan, with no geographic projection.
Compared with similar charts
Against a bar chart of regions: the bar chart ranks and compares values accurately. The map shows where the values are and which ones sit together.
Choropleth against bubble map: shade a choropleth by a rate, and size bubbles by a total. Bubbles also keep small regions visible.
Against a heatmap: a heatmap puts the regions in a grid with equal cells, so every region gets the same space.
Against a table: the table gives exact values and sorts. Pair it with the map whenever the values matter.