Comparable choropleth maps — Recipe (QGIS)

Tool-bound implementation of the Comparable choropleth maps (Task) Task Schema in QGIS 4.2. Worked example: Dwelling size by construction period in Copenhagen (Example).

Data you need

Role	Form	Level of measurement	Notes
Aggregated zones, one layer per group	polygons	ratio value field and a count field	from Aggregating points to zones (Task), with the same zones and minimum count
Shared style (output)	a .qml file	—	the same classes and colours for every map

Look at all groups’ values (Task Schema step 2)

Open the statistics panel on the value field of each layer and note the combined range.

One set of classes, with a class for too few (steps 3–4)

Layer Properties › Symbology › Categorized with an expression as Value (the ε button), so that zones with too few points get their own class:

CASE WHEN coalesce("<count field>", 0) < <minimum> THEN 'Too few'
     WHEN "<value field>" < <break 1> THEN 'under <break 1>'
     WHEN "<value field>" < <break 2> THEN '<break 1>–<break 2>'
     …
     ELSE '<last break> or more' END

Press Classify, put the categories in order, and give them one sequential colour scheme, with grey for “Too few”. With Graduated symbology (Tool - QGIS) instead, set the boundaries by hand (Fixed Interval or typed values) and add a rule for too few.

Same style on every map (step 5)

Style › Save Style… to a .qml file, then Style › Load Style… on the other layers; or Styles › Copy Style and Paste Style.

Compose (step 6)

Print layout (Tool - QGIS): a title that names the group, the legend, a scale bar, a north arrow and a source line on each map; the same extent and scale on all of them.

Checked in QGIS 4.2.2, October 2026.