Heatmaps and Densities

Heatmaps and density charts answer where values concentrate across two dimensions. A matrix uses explicit row and column categories or intervals. Contours and spatial bins summarize a continuous point field. Choose the representation that preserves the question instead of defaulting to one color per raw observation.

Choose the comparison

Reader question	Start with
How many observations fall in each quantitative x-y interval?	Binned quantitative heatmap
What smooth regions enclose similar point density?	Density contours
Where are dense clusters while retaining local bin shape?	Hexagonal bins
What value belongs to each pair of named categories?	An ordinal cell matrix

Data and Channels explains interval and color channels. Legends and Color covers continuous color meaning and accessible legend design.

Aggregate into quantitative cells

A two-dimensional binned heatmap makes density bounded: the number of rendered cells depends on the chosen grid, not directly on the number of raw points.

Prepare explicit x and y interval endpoints plus the aggregate value. Use thresholds that are stable across comparable views and a color domain that states whether absolute count or normalized density is being shown. See Scales and D3 for binning and scale ownership.

Summarize a continuous field with contours

Density contours turn many points into nested level sets. They are useful for revealing cluster shape and overlap when raw dots would occlude one another.

Bandwidth and thresholds change the visible shape. Treat them as analytical parameters, keep them stable for comparisons, and explain them when they affect interpretation. This case uses an optional custom mark boundary described in Custom Marks and Renderers.

Retain local structure with hexagonal bins

Hexagonal bins aggregate nearby points in pixel space and encode each bin's count or statistic. They provide a compact alternative when a rectangular grid would impose stronger horizontal and vertical edges.

Pixel-space binning is responsive work: a changed container changes the spatial layout. Keep that preparation aligned with the chart's measured inner bounds. Responsive Charts defines the measurement lifecycle; Dot and Hexagon Marks defines the rendered mark.

Production checks

State whether color represents count, proportion, rate, or another aggregate.
Choose a sequential, diverging, or threshold scale that matches the data semantics.
Keep missing, zero, and out-of-domain cells visually distinct.
Add direct labels only when cells remain large enough to read. A labeled ordinal matrix is demonstrated in Themes and Styling.
Prefer aggregation over rendering an unbounded raw point layer. See Large Data.