Heatmap Design Guide AI Prompt

Design an effective heatmap for this data.

Data: {{data_description}}
Rows: {{row_dimension}}
Columns: {{column_dimension}}
Values: {{value_metric}}

1. When to use a heatmap:
   - When the combination of two categorical or ordinal dimensions determines an outcome
   - When there are too many cells for individual bar charts
   - When the pattern across the full matrix is the insight (not individual values)
   - Classic uses: hour-of-day × day-of-week, correlation matrix, cohort retention

2. Color scale selection:
   - Sequential (one direction): light = low, dark = high. Use for: volume, count, positive metrics.
   - Diverging (two directions from midpoint): use for: correlation (-1 to +1), deviation from target (negative to positive), profit/loss.
   - Categorical: only if cells represent categories, not values

   Color scale specifics:
   - For retention or positive rates: white/light → brand color
   - For correlation matrices: blue → white → red (standard in statistics)
   - For profit/loss: red → white → green
   - Always: make the color scale legend visible with clear breakpoints labeled

3. Ordering rows and columns:
   - Do NOT use alphabetical order unless that is meaningful
   - Order by: magnitude (row total descending), natural order (Mon–Sun, Jan–Dec), or hierarchical clustering
   - Hierarchical clustering: groups similar rows and columns together, revealing pattern blocks

4. Cell annotations:
   - Add the value in each cell when: precision matters AND the matrix is small (< 100 cells)
   - For large matrices: use color only, with hover tooltips for exact values
   - Number format: 1 decimal for percentages; abbreviate large numbers (1.2M)
   - Text color: use dark text on light cells, light text on dark cells (auto-switch at midpoint)

5. Size and aspect ratio:
   - Square cells: ideal for correlation matrices where both dimensions are the same concept
   - Rectangular cells: for matrices where row and column dimensions differ substantially
   - Target: cells large enough to read the annotation (minimum 30×30px for annotated cells)

6. Marginal summaries:
   - Add row totals (right side) and column totals (bottom)
   - Use a lighter shade or a bar chart strip for marginals
   - This helps interpret relative importance of each row/column

Return: color scale specification, ordering recommendation, annotation rules, and marginal summary design.