Agent skill: Edward Tufte's data visualization principles for clean, honest, high-data-ink-ratio charts. Recharts, ECharts, Chart.js, matplotlib, Plotly, D3/SVG.
SaferSkills independently audited tufte-data-viz (Agent Skill) and scored it 100/100 (green). The audit ran 55 deterministic rules across Security, Supply Chain, Maintenance, Transparency, and Community; it found 0 high-severity and 0 lower-severity findings. The full rule-by-rule trace and per-finding evidence are below. Free, methodology-open.
Findings & checks · 0 flagged
Every scanned point with the score it earned and what moved between them.
First recorded scan — no prior version to compare against.
The primary manifest — the file an agent reads to learn what this artifact does.
Apply Edward Tufte's principles whenever generating or reviewing code that renders data visually. This skill covers chart generation, not slide/presentation design.
Follow these steps in order when creating any chart:
Before writing code, determine:
Review the rules below. Every rule is a default — deviate only when the user explicitly requests otherwise.
Use the Library quick reference table to find the essential overrides for the target library. For complete code examples and helper functions, read ONE rule file from rules/ matching the library.
Run through the validation checklist at the bottom of this file before presenting the chart.
Rules 1–14 cover static principles; 15–19 extend them for screens; 20–22 address content and formatting.
No chart should have top or right axis lines, borders, or spines. The bottom and left axes are sufficient. Top and right lines are pure chartjunk.
Label each data series directly — at the endpoint of a line, on or beside a bar, next to a cluster. Remove the <Legend> component entirely. If there is only one series, the chart title provides that context; no label is needed.
The default is zero gridlines. For static charts where users need to read precise values, add horizontal-only gridlines at very low opacity (0.08–0.12). For interactive charts, prefer a contextual crosshair on hover instead (see rule 15). Never add vertical gridlines.
Axis lines should span only the range of the data, not from zero to some arbitrary maximum. The axis starts at (or near) the minimum data value and ends at the maximum.
No perspective, no depth, no shadows on chart elements. Two-dimensional data gets two-dimensional representation.
Default to a horizontal bar chart sorted by value. If the user explicitly asks for a pie chart: maximum 4 slices, 2D only, start at 12 o'clock, direct percentage labels on each slice.
Charts should be approximately 50% wider than tall. Standard sizes: 600x400, 750x500, 900x600. Exception: sparklines and small multiples may be more compact.
The default data series color is medium gray (#666). Use a single accent color to highlight the most important series or data point. Never use more than 4 distinct colors. Choose the right palette type: categorical (4-color muted) for unordered groups, sequential (single-hue ramp) for ordered magnitude, diverging (two-hue from center) for deviation from a midpoint. See rules/typography-and-color.md for hex values.
Light mode: #fffff8. Dark mode: #151515. Never use pure white (#ffffff) or pure black (#000000).
Use serif fonts for data labels, annotations, and chart titles: "ET Book", "Palatino Linotype", Palatino, "Book Antiqua", Georgia, serif. Sans-serif (system-ui, sans-serif) is acceptable only for small axis tick labels (11-12px).
Two y-axes on one chart create false implied correlations. Use small multiples instead — two charts stacked vertically with shared x-axis.
If the data contains a peak, trough, inflection point, or event boundary, add a text annotation pointing to it directly on the chart. Place annotations in the nearest clear space — offset from the data point with a short leader line if needed. When multiple annotations compete for space, keep only the most important; move others to a footnote or tooltip.
Include at least one reference element: a reference line (average, target, prior period), a shaded band, or a second series. A chart showing one line with no context fails the "Compared to what?" test.
Tooltips should be plain text with the data value and label. No colored background, no border, no arrow pointer, no shadow.
Default to the Tufte-clean overview — high data-ink, minimal chrome. Layer details through hover, tap, and click (values, annotations, comparisons). Don't frontload everything onto a single static view. A contextual crosshair on hover replaces permanent gridlines.
3:1 contrast ratio minimum for chart elements against their background; 4.5:1 for text in charts. Never use color as the sole differentiator — pair with shape, pattern, or direct label. Provide a text alternative for every chart (aria-label with key finding, or companion data table). Interactive charts must be keyboard-navigable.
Charts must have a responsive strategy — fluid (percentage width + viewBox), adaptive (breakpoint-based layout changes), or hybrid. At narrow viewports, change chart type or layout (horizontal bars for categories, reduced tick density, abbreviated labels), don't just shrink.
Transitions for data changes (sorting, filtering, time progression) are good — they help the viewer track transformations. Gratuitous entrance animations, bouncing, and decorative motion are chartjunk. Duration: 200–500ms, ease-out. Always respect prefers-reduced-motion.
Design both light and dark palettes intentionally. Never invert colors. Reduce saturation in dark mode (bright colors "vibrate" on dark backgrounds). Respect prefers-color-scheme. Use semantic color tokens (--tufte-bg, --tufte-text, --tufte-series-default) so charts adapt automatically.
The chart title states the key insight, not the axis description. "Revenue Surged 23% in Q3" not "Revenue by Quarter, 2024". The subtitle can provide context ("vs. prior year, USD millions"). If the data has no clear finding, the chart may not be needed (see rule 22).
Abbreviate large numbers: $1.2M not $1,200,000. Use thousand separators for mid-range numbers (12,450 not 12450). Match decimal precision to significance (don't show $4.2391M when $4.2M suffices). Right-align numbers in tables. Use consistent units and state them once (in the axis label or title), not on every data point.
If the data is 1–2 numbers, write a sentence with inline context ("Revenue was $4.2M, up 23% from Q2"). If the data is a simple ranking of 3–5 items, consider a table. Charts earn their space by revealing patterns, trends, or distributions that text and tables cannot. A chart of two bars is almost always worse than a sentence.
The universal rules above are sufficient for most charts. For complete code examples and library-specific helpers, read the appropriate rule file from the rules/ directory in this skill's folder. Only read ONE rule file per task.
| Library | Rule file to read | Essential config (apply even without reading the file) |
|---|---|---|
| Recharts | rules/recharts.md | <CartesianGrid stroke="none" />, remove <Legend />, <YAxis axisLine={false} tickLine={false} />, <Line dot={false} strokeWidth={1.5} /> |
| ECharts | rules/echarts.md | splitLine: { show: false }, legend: { show: false }, grid: { show: false }, use endLabel on series |
| Chart.js | rules/chartjs.md | grid: { display: false }, border: { display: false }, plugins.legend.display: false, use chartjs-plugin-datalabels |
| matplotlib | rules/matplotlib.md | spines['top'].set_visible(False), spines['right'].set_visible(False), spines['bottom'].set_bounds(min, max), font.family: serif |
| Plotly | rules/plotly.md | showgrid=False, showlegend=False, plot_bgcolor='#fffff8', zeroline=False |
| D3/SVG/HTML | rules/svg-html.md | .domain { display: none }, no <rect> backgrounds, stroke-opacity: 0.1 for any gridlines |
| Type | Key settings |
|---|---|
| Line | 1.5–2px stroke, dot={false} unless <7 points (then r=2), direct label at rightmost point |
| Bar | Prefer horizontal for categories, sort by value descending, direct value labels, #7a7a7a default fill |
| Scatter | Gray dots #999 r=3, highlight key cluster/outlier with accent, regression line if meaningful (dashed, thin) |
| Time series | Label events on chart ("Recession", "Launch"), range-frame x-axis, YoY via opacity (current solid, prior 30%) |
| Small multiples | Same scale ALL panels, shared axis labels (x on bottom row, y on left column), no panel borders |
| Sparklines | ~80x20px, no axes/labels/gridlines, min/max dots r=1.5, embed inline in text or table cells |
| Data tables | No zebra striping, whitespace + thin rules every 3–5 rows, right-align numbers, font-feature-settings: 'onum' 1 |
| Slopegraph | Before/after categories, label both endpoints (value + name), gray default + highlight key slopes |
| Area | Prefer lines. If area: fillOpacity 0.03–0.08, no gradient, direct labels at endpoints |
| Stacked bar | Avoid — use small multiples instead. If forced: sort by total, direct labels per segment, max 4 segments |
| Heatmap | Sequential or diverging palette only, value labels in cells, companion data table for accessibility |
For small multiples, sparklines, and slopegraph implementation patterns, see rules/small-multiples-sparklines.md.
| Token | Light | Dark |
|---|---|---|
| Background | #fffff8 | #151515 |
| Text | #111 | #ddd |
| Text secondary | #666 | #999 |
| Axis/rule | #ccc | #444 |
| Grid (if used) | #eee (8-12% opacity) | #333 |
| Default series | #666 | #999 |
| Highlight | #e41a1c | #fc8d62 |
Categorical (max 4): #4e79a7 steel blue · #f28e2b tangerine · #e15759 coral · #76b7b2 sage
Font stacks in rule 10. For full palettes (sequential, diverging), font loading, and old-style figures, see rules/typography-and-color.md.
When reviewing existing chart code, check for: legends (→ direct labels), pie charts (→ horizontal bars), 3D effects (→ flat 2D), dual y-axes (→ small multiples), heavy gridlines (→ remove or 0.1 opacity), rainbow palettes (→ gray + accent), gauge widgets (→ number + sparkline), gradient fills (→ solid color), rotated labels (→ flip axes or abbreviate), pure white/black backgrounds (→ #fffff8/#151515), hover-only information (→ tap/focus fallback), missing text alternatives (→ aria-label), color-only encoding (→ add shape/pattern).
For the full table with per-library detection patterns and one-liner fixes, see rules/anti-patterns.md.
Before presenting any chart, verify:
#fffff8 (light) or #151515 (dark), not pure white/black#666); color used only for emphasisprefers-reduced-motionLibrary rules (read ONE per task): rules/recharts.md, rules/echarts.md, rules/chartjs.md, rules/matplotlib.md, rules/plotly.md, rules/svg-html.md — complete code examples, helpers, and theme registrations.
Cross-cutting (read when specifically needed):
rules/interactive-and-accessible.md — progressive disclosure, WCAG, responsive, animation, dark moderules/typography-and-color.md — font loading, full palette tables, old-style figuresrules/anti-patterns.md — per-library detection heuristics and fixesrules/small-multiples-sparklines.md — layout patterns for small multiples, sparklines, slopegraphsWorking examples in examples/ — one per library, plus an inline SVG sparkline.
~30 seconds. Free. No account. Every finding cites a rule and a line of evidence.