Chart Pattern · Reference

Heatmap

2D categorical or temporal · color = magnitude

Daily activity by day-of-week × hour-of-day

Each cell = one (day × hour) bucket · darker = higher activity · pattern: peak Tue–Thu, 10am–4pm

✓ When to use

  • Two categorical dimensions you want to compare across
  • Calendar patterns (hour × day, day × month)
  • Correlation matrix or risk likelihood × impact

✗ Avoid when

  • Magnitude needs to be precisely read — bars are more accurate
  • One dimension is continuous — use a line per category
  • Most cells are empty — sparse data wastes the grid

★ Best practice

  • Use a perceptually uniform colorscale (Viridis, Blues, etc.)
  • Always include a color legend with units
  • Label cell values when grid is small enough (≤8×8)
  • Sort axes by hierarchy or magnitude — never alphabetical