Data Visualization Skill
Chart selection guidance, Python visualization code patterns, design principles, and accessibility considerations for creating effective data visualizations.
Chart Selection Guide
Choose by Data Relationship
When NOT to Use Certain Charts
- Pie charts: Avoid unless <6 categories and exact proportions matter less than rough comparison. Humans are bad at comparing angles. Use bar charts instead.
- 3D charts: Never. They distort perception and add no information.
- Dual-axis charts: Use cautiously. They can mislead by implying correlation. Clearly label both axes if used.
- Stacked bar (many categories): Hard to compare middle segments. Use small multiples or grouped bars instead.
- Donut charts: Slightly better than pie charts but same fundamental issues. Use for single KPI display at most.
Python Visualization Code Patterns
Setup and Style
Line Chart (Time Series)
Bar Chart (Comparison)
Histogram (Distribution)
Heatmap
Small Multiples
Number Formatting Helpers
Interactive Charts with Plotly
Design Principles
Color
- Use color purposefully: Color should encode data, not decorate
- Highlight the story: Use a bright accent color for the key insight; grey everything else
- Sequential data: Use a single-hue gradient (light to dark) for ordered values
- Diverging data: Use a two-hue gradient with neutral midpoint for data with a meaningful center
- Categorical data: Use distinct hues, maximum 6-8 before it gets confusing
- Avoid red/green only: 8% of men are red-green colorblind. Use blue/orange as primary pair
Typography
- Title states the insight: "Revenue grew 23% YoY" beats "Revenue by Month"
- Subtitle adds context: Date range, filters applied, data source
- Axis labels are readable: Never rotated 90 degrees if avoidable. Shorten or wrap instead
- Data labels add precision: Use on key points, not every single bar
- Annotation highlights: Call out specific points with text annotations
Layout
- Reduce chart junk: Remove gridlines, borders, backgrounds that don't carry information
- Sort meaningfully: Categories sorted by value (not alphabetically) unless there's a natural order (months, stages)
- Appropriate aspect ratio: Time series wider than tall (3:1 to 2:1); comparisons can be squarer
- White space is good: Don't cram charts together. Give each visualization room to breathe
Accuracy
- Bar charts start at zero: Always. A bar from 95 to 100 exaggerates a 5% difference
- Line charts can have non-zero baselines: When the range of variation is meaningful
- Consistent scales across panels: When comparing multiple charts, use the same axis range
- Show uncertainty: Error bars, confidence intervals, or ranges when data is uncertain
- Label your axes: Never make the reader guess what the numbers mean
Accessibility Considerations
Color Blindness
- Never rely on color alone to distinguish data series
- Add pattern fills, different line styles (solid, dashed, dotted), or direct labels
- Test with a colorblind simulator (e.g., Coblis, Sim Daltonism)
- Use the colorblind-friendly palette:
sns.color_palette("colorblind")
Screen Readers
- Include alt text describing the chart's key finding
- Provide a data table alternative alongside the visualization
- Use semantic titles and labels
General Accessibility
- Sufficient contrast between data elements and background
- Text size minimum 10pt for labels, 12pt for titles
- Avoid conveying information only through spatial position (add labels)
- Consider printing: does the chart work in black and white?
Accessibility Checklist
Before sharing a visualization:
- Chart works without color (patterns, labels, or line styles differentiate series)
- Text is readable at standard zoom level
- Title describes the insight, not just the data
- Axes are labeled with units
- Legend is clear and positioned without obscuring data
- Data source and date range are noted
