How to build a choropleth map in PowerPoint
A choropleth is five colors and a legend. Almost all of the work is in choosing the breaks and joining the numbers to the shapes correctly; the coloring takes a minute once those two are settled.
What a choropleth actually needs
A choropleth is a map filled by class: one color per band of values, with a legend that says what each band means. It needs three things and nothing else — areas that can be filled separately, a value for each area, and a set of class breaks that turns those values into five colors.
PowerPoint supplies the fills. The one thing it cannot supply is areas that fill independently, unless the map arrives as shapes rather than as a picture. That is the whole prerequisite, and everything below assumes it.
Choose the breaks before the colors
The commonest mistake is picking five shades first and then squeezing the data into them. The breaks decide what the map says; the shades only make it readable.
- Equal intervals — the range divided into equal steps. Shows magnitude honestly, and can leave a class empty when the data is skewed, which is most of the time.
- Quantiles — the same number of areas in each class. Good for rank, and quiet about how large the gap between the top and the rest is.
- Natural breaks — breaks placed where the data clusters. The best picture of the data's own shape, and the hardest to explain in a footnote.
Pick one, write it in the slide notes, and keep it across quarters. A map whose breaks move every quarter makes last quarter's map unreadable beside this one.
Five classes is a limit, not a style
On a projector, in a room with the lights on, five shades of one hue are about the limit of what a reader can separate. Seven is possible in print. Beyond that the legend becomes a lookup table and the map becomes a decorated table.
The alternative to more classes is fewer: two classes — above target and below — answer most business questions more clearly than five do.
Color from the file's own palette
A template that ships three themes also ships the five-step palette for each theme, on its data slide. Using those swatches rather than picking colors from the standard palette is what keeps a choropleth looking like part of the deck instead of something pasted into it.
Joining numbers to shapes by code
Names are not a join key. More than thirty states have a Washington County, and a spreadsheet that matches on “Washington” will color one of them and leave the rest blank. Every shape in a file built from official boundary data carries the national code for its area: a Census GEOID in the United States, a GSS code in the United Kingdom, a GB code in China, a national code in Spain.
- Open the data slide, which lists every area and its code.
- Paste your values in the column beside the codes, in the same order or sorted the same way.
- Classify, then fill each class from the swatches.
The legend and the labels
A choropleth without a legend is a colored map. The legend needs the
class breaks written as ranges — 0–4.9%,
5–9.9% — rather than as five unnamed squares,
and it needs the unit. If the value is a rate, say what it is a rate
of.
Labels are the other half. On a national county map there is no room for 3,225 names and none are needed: label the ten largest areas, or the ones the story is about. The rest of the map carries the pattern.
A worked example
Take a state's 254 counties, a value per county, and five classes on quantiles. Paste the values beside the codes, sort, cut into five equal groups, fill each group from the swatches, and write the five ranges into the legend. The result is one slide: no add-in, no chart, and a map that is recolored next quarter by changing fills rather than by rebuilding anything.
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