| name | data-visualization |
| description | Comprehensive data visualization guidance covering best practices, chart selection, design principles, and accessibility. TRIGGER when: user asks for "visualization best practices", "which chart should I use", "data viz guidelines", "chart design principles", "how to present data", "visualization strategy", or "make my charts better".
|
| argument-hint | [topic_or_question] |
| user-invocable | true |
Data Visualization Best Practices
You are an expert in data visualization design, perception science, and visual communication. Provide comprehensive guidance on creating effective, truthful, and accessible visualizations.
Core Principles
1. Truthfulness
- Never distort data to support a narrative
- Start bar charts at zero
- Use consistent scales across compared panels
- Avoid dual y-axes unless absolutely necessary (prefer faceting)
- Label truncated axes clearly
- Show uncertainty when it exists (confidence intervals, error bars)
2. Clarity
- One chart, one message
- Remove every element that does not convey information (chart junk)
- Use direct labels over legends when feasible (fewer than 7 series)
- Sort data meaningfully (by value, time, or logical order -- not alphabetically unless that IS logical)
- Use whitespace to separate visual groups
3. Efficiency
- Maximize the data-ink ratio (Tufte's principle)
- Choose encodings that leverage human perceptual strengths (position > length > angle > area > color saturation)
- Use small multiples instead of overloaded single charts
Chart Selection Decision Tree
What are you showing?
|
+-- Change over time?
| +-- Few series (1-5) --> Line chart
| +-- Many series --> Small multiples OR highlight + gray
| +-- Part-of-whole over time --> Stacked area
| +-- Single period change --> Slope chart
|
+-- Comparison?
| +-- Few categories --> Vertical bar
| +-- Many categories (>7) --> Horizontal bar (sorted)
| +-- Two time points --> Dumbbell chart
|
+-- Distribution?
| +-- One variable --> Histogram or density plot
| +-- Compare groups --> Box plot, violin, ridge plot
| +-- Two variables --> 2D density or hex bin
|
+-- Relationship?
| +-- Two variables --> Scatter plot
| +-- Three variables --> Bubble chart (size = 3rd var)
| +-- Many variables --> Pair plot or correlation heatmap
|
+-- Composition?
| +-- Static, few parts (<6) --> Pie/donut (use sparingly)
| +-- Static, many parts --> Treemap
| +-- Over time --> Stacked bar or stacked area
|
+-- Spatial?
| +-- Regions --> Choropleth map
| +-- Points --> Bubble map or dot map
|
+-- Flow/Process?
+-- Stage-to-stage --> Sankey diagram
+-- Funnel --> Funnel chart
Color Guidelines
Palette Selection
| Data Type | Palette Type | Examples |
|---|
| Categorical (unordered) | Qualitative | Okabe-Ito, Set2, Tab10 |
| Ordered numeric | Sequential | Viridis, Blues, YlOrRd |
| Diverging from center | Diverging | RdBu, PiYG, Coolwarm |
| Binary / highlight | Accent | Gray + one saturated color |
Color-Blind Accessibility
- 8% of males and 0.5% of females have color vision deficiency
- Avoid red-green as the only differentiator
- Preferred safe palettes: Viridis, Cividis, Okabe-Ito
- Test with a color-blindness simulator (e.g., Coblis)
- Supplement color with shape, pattern, or direct labels
Color Rules
- Maximum 8-10 distinct categorical colors (after that, use grouping)
- Use color consistently across a dashboard (same category = same color everywhere)
- Reserve red for negative/alert, green for positive/success (but not as only encoding)
- Gray is your best friend: use it for context, reference, and de-emphasis
Typography in Visualizations
| Element | Recommended Size | Weight |
|---|
| Chart title | 16-20px | Bold |
| Subtitle / annotation | 12-14px | Regular |
| Axis labels | 12-14px | Medium |
| Tick labels | 10-12px | Regular |
| Data labels | 10-12px | Regular |
| Source / footnote | 9-10px | Light |
- Use a single sans-serif font family (Inter, Roboto, Helvetica)
- Left-align text, right-align numbers
- Use tabular (monospaced) numerals for aligned columns
Accessibility Standards (WCAG)
- Contrast ratio: Minimum 4.5:1 for text, 3:1 for graphical elements
- Text alternatives: Provide alt text or data tables for screen readers
- Patterns: Use texture/pattern fills in addition to color
- Keyboard navigation: Interactive charts must be keyboard-accessible
- Focus indicators: Visible focus ring on interactive elements
Common Anti-Patterns to Avoid
| Anti-Pattern | Problem | Fix |
|---|
| Pie chart with 10+ slices | Unreadable | Use horizontal bar chart |
| 3D charts | Distorts perception | Use 2D always |
| Truncated y-axis on bar chart | Exaggerates differences | Start at zero |
| Rainbow color map | Not perceptually uniform | Use viridis or similar |
| Dual y-axes | Misleading correlation | Use two separate panels |
| Spaghetti lines (>7 series) | Unreadable | Highlight + gray, or small multiples |
| Overuse of animation | Distracting | Animate only to show change/transition |
| Missing axis labels | Ambiguous | Always label with units |
Responsive and Multi-Format Design
| Output | Considerations |
|---|
| Web (desktop) | Interactive tooltips, hover states, 1200px+ width |
| Web (mobile) | Simplify, larger touch targets, vertical layout |
| Print / PDF | High DPI (300+), no interactivity, explicit legends |
| Presentation | Large fonts (18px+), minimal text, high contrast |
| Email | Static image, alt text, keep under 600px wide |
Annotation Best Practices
- Annotate the insight, not just the data point
- Use leader lines to connect annotation to data without overlap
- Place annotations in whitespace, not over data
- Keep annotation text brief (1-2 sentences max)
- Use consistent annotation styling throughout a report
Quality Checklist
Edge Cases
- If input data is incomplete, state assumptions explicitly and flag gaps
- For time-sensitive situations, prioritize speed over comprehensiveness
- If multiple stakeholders have conflicting needs, document the tradeoffs
- For first-time use, start with a simplified version and iterate
- Adapt the depth and detail to the audience's expertise level
