| name | canon-charts |
| description | Use when designing, auditing, or refactoring charts, graphs, data visualizations, dashboards, sparklines, or any visual representation of data. Covers chart-type selection, color, axes, labels, tooltips, accessibility, animation, and anti-patterns like 3D pies and dual-axis charts. Trigger when the user mentions chart, graph, visualization, dashboard, bar chart, line chart, pie chart, or sparkline. |
CANON · Charts
A chart is a claim. If the reader can't extract the claim in five seconds, the chart failed regardless of how pretty it is.
Pick the right chart type
| Goal | Chart type |
|---|
| Compare categories | Bar chart (horizontal if labels are long) |
| Show trend over time | Line chart |
| Show part-of-whole, ≤ 5 slices | Pie or donut (used sparingly) |
| Show part-of-whole, > 5 slices | Stacked bar or treemap |
| Show distribution | Histogram, box plot |
| Show correlation | Scatter plot |
| Show change across two points | Dumbbell chart, slope chart |
| Single key metric with trend | Big number + sparkline |
| Show composition over time | Stacked area |
Axis rules
- Y-axis should start at zero for bar charts. Always. Truncating bar axes inflates differences and misleads.
- Line charts may use a non-zero baseline if the data range is narrow and a zero-start would flatten the signal — in that case, explicitly note the baseline.
- X-axis time: left → right, always. Never right → left.
- Label axes clearly. "Revenue ($)" beats "$".
- Tick marks: 4–7 per axis. More = visual noise. Fewer = hard to read.
- Grid lines: light, thin, optional. Full grids often hurt. Horizontal-only grid for vertical bar charts.
Color in charts
Follow canon-color for palette. Chart-specific rules:
- Categorical (qualitative): distinct hues, roughly equal perceptual weight. 5–8 max. Beyond 8, reorganize the data.
- Sequential (ordinal): single-hue light-to-dark. Works for ordered data (low → high).
- Diverging: two hues from neutral midpoint. Use for data with a meaningful zero or center (profit/loss, above/below average).
Avoid rainbow palettes — they imply order where there is none and fail for colorblind users.
Avoid red/green as the only signal. 8% of men are red-green colorblind. Pair with shape or position.
Test palettes against deuteranopia, protanopia, tritanopia simulators.
Labels over legends
If you can label data directly, do it. Legends force the eye back and forth.
Bad:
[chart with color swatches]
Legend: ■ Q1 ■ Q2 ■ Q3
Better:
[chart with "Q1 $2.1M" labeled next to the bar]
When legend is required, place it top-left or aligned with the chart title, not bottom-right.
Tooltips
- Trigger: hover (desktop), tap (mobile). Keyboard focus must also trigger.
- Content: the exact value, the label, optional context ("+12% vs Q3").
- Position: above the point, offset 8–12px. Never covers the cursor.
- Follow the cursor for line/scatter; snap to nearest data point.
- Fade: 120–180ms on enter, 100ms on exit.
Accessibility
- Every chart needs a text alternative:
aria-label on the chart container, or <figcaption> summarizing the finding.
- Provide a data table alternative — either visible (toggle) or screen-reader-only. WCAG 1.1.1.
- Focusable data points with keyboard nav in complex charts. Arrow keys move between points, Enter/Space reads the value.
- Color is never the only signal. Add pattern fills, shapes, or labels.
- Don't animate past
prefers-reduced-motion.
Animation
| Purpose | Allowed? | How |
|---|
| First-paint entrance | Yes | 400–600ms ease-out, each bar/line staggered 30–60ms |
| Data update | Yes | 300–400ms ease-in-out, interpolate values |
| Decorative motion (pulsing, rotating) | No | Serves no purpose, distracts |
| Tooltip | Yes | 120–180ms fade |
Honor prefers-reduced-motion: reduce with an instant-draw fallback.
Labels and numbers
- Use
font-variant-numeric: tabular-nums so digit widths align.
- Thousands separators (1,234 not 1234) unless the space is desperate.
- Round intelligently: 2.4M not 2,400,000 for labels; precise for tooltips.
- Currency symbol attached, no space:
$1.2M not $ 1.2 M.
- Percentages without decimal where precision isn't critical:
12% not 12.00%.
Sizing and aspect ratio
- Line charts: 16:9 or 3:1 (wide for trends).
- Bar charts: 4:3 or 1:1, enough vertical room for labels.
- Minimum readable size: 320×240px. Smaller → sparkline territory.
- Sparkline: 80–160px wide, 24–40px tall, no axes, single sparkline per row.
Dashboards
- Most important metric: top-left (F-pattern entry).
- Hierarchy: big numbers > charts > tables.
- Consistent card heights in a row.
- Timeframe controls visible and global.
- Max 8 tiles per dashboard screen — more needs drill-down, not density.
Common chart bugs
| Bug | Fix |
|---|
| Bars truncated at top | Add 10% headroom above max data point |
| Overlapping X-axis labels | Rotate 30–45°, or use horizontal bars |
| Same color for positive and negative | Use a diverging palette |
| Time axis not in order | Sort by date always |
| Tooltip flickering | Add 100ms hover delay, use pointer-events: none inside the chart plot |
| Legend wraps awkwardly | Break to new line at fixed width, or convert to direct labels |
Anti-patterns
| Anti-pattern | Why it fails |
|---|
| 3D pie charts | Distort perception of slice sizes |
| Dual-axis line charts | Misleading; two different scales suggest correlation that doesn't exist |
| Pie chart with 8+ slices | Unreadable; use stacked bar |
| Rainbow palette for categorical data | Implies order, colorblind fail |
| Red and green as only signal | 8% colorblind population fails to distinguish |
| Truncated Y-axis on bar chart | Exaggerates differences, misleads |
| Chart with no title | Users guess the point |
| Chart with no units | "$2.4" — millions? thousands? |
| Pulsing / rotating decoration | Distraction, triggers motion sensitivity |
| Tooltip covering the data point | Can't re-check the source |
| Animation on every data update | Chart thrashes in live dashboards |
Decision tree
What's the user trying to learn?
Compare values across a few categories → Bar chart
Spot trend over time → Line chart
Composition of one whole → Donut (≤ 5 slices) or stacked bar (> 5)
Correlation between two variables → Scatter
Distribution → Histogram or box plot
How did N series change between two snapshots → Slope or dumbbell
One key number → Big number + optional sparkline
Audit checklist
Sources
- Edward Tufte · The Visual Display of Quantitative Information
- Stephen Few · Information Dashboard Design
- Cole Nussbaumer Knaflic · Storytelling with Data
- WCAG 2.2 · 1.1.1 Non-text Content, 1.4.1 Use of Color, 2.1.1 Keyboard
- Viz Palette · colorblind-safe palettes
- Material Design 3 · Data visualization
