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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

• Chart type is appropriate for the data relationship
• Title answers "what does this show?"
• All axes have labels with units
• Color palette is color-blind safe
• No chart junk (3D, unnecessary gridlines, decorative elements)
• Data-ink ratio is high
• Numbers are formatted with appropriate precision
• Annotations highlight the key insight
• Source is credited
• Accessibility standards are met (contrast, alt text)
• Chart works at intended output size and format

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