# Heatmap — Situational Evidence

Use to encode magnitude across a dense two-dimensional matrix. Prefer a table when readers primarily need exact values and the matrix is small.

## Required

- Choose a sequential map for ordered magnitude or a diverging map for values around a meaningful midpoint.
- Include a labeled colorbar with units, meaningful endpoints, and the midpoint when diverging.
- Distinguish zero, missing, below-detection, and not-applicable when those states occur.
- Preserve meaningful row and column order; disclose clustering or reordering.
- Size cells and labels so the matrix remains traceable at final output size.

## Allowed choices

- Dense cells may be borderless; low-contrast adjacent stops may carry rank.
- Add values inside cells only when the matrix is small and every label meets contrast.
- Cluster rows or columns when similarity is the analytical question and the method is available.
- Use a neutral missing-data mark outside the quantitative ramp.

## Forbidden

- Do not use a diverging map without a meaningful center.
- Do not map zero and missing to the same appearance.
- Do not outline every cell when gutters or the matrix geometry already separate them.
- Do not use rainbow or ad hoc palettes.