Blog Chart: Built-In SVG Data Visualization

Generates dark-mode-compatible inline SVG charts for blog posts. Invoked internally by blog-write and blog-rewrite when chart-worthy data is identified. Not a standalone user-facing command.

Styling source of truth: skills/blog/references/visual-media.md

For supported chart types, prefer the deterministic CLI:

python3 skills/blog-chart/scripts/generate_chart_svg.py --input chart.json --output chart.html --json

Input Format

The writer or researcher passes a chart request:

Chart Request:
- Type: horizontal bar
- Title: "Quarterly Signups by Product"
- Data: Product A 420, Product B 315, Product C 180
- Source: [Verified source], [publication date]
- Platform: mdx (or html)

Chart Type Selection

Select based on the data pattern. Prefer chart type diversity, but repeat a type when comparability or reader comprehension clearly benefits.

Styling Rules (Non-Negotiable)

All charts must work on both dark and light backgrounds:

Text elements:     fill="currentColor"
Grid lines:        stroke="currentColor" opacity="0.08"
Axis lines:        stroke="currentColor" opacity="0.3"
Background:        transparent (no fill on root SVG)
Subtitle text:     fill="var(--chart-muted, currentColor)"
Source text:        fill="var(--chart-muted, currentColor)"
Label text:        fill="currentColor" opacity="0.8"

Set --chart-muted to an accessible text token in the host theme. If no token exists, use #4b5563 on light backgrounds and #d1d5db on dark backgrounds. Do not rely on low-opacity source or subtitle text for visible attribution.

Color Palette

For text inside approved colored elements: use fill="#111827" with fontWeight="800". Only use white text after checking the contrast ratio is at least 4.5:1 against that specific fill color.

Do not rely on color alone. Add direct labels, patterns, line dashes, marker shapes, or legend text so colorblind readers can distinguish series.

Standard SVG Shell (HTML)

<svg
  viewBox="0 0 560 380"
  style="max-width: 100%; height: auto; font-family: 'Inter', system-ui, sans-serif"
  role="img"
  aria-labelledby="chart-title chart-desc"
>
  <title id="chart-title">Chart Title</title>
  <desc id="chart-desc">Description for screen readers with all key data points and source</desc>

  <!-- Chart content -->

  <text x="280" y="372" text-anchor="middle" font-size="10" fill="var(--chart-muted, currentColor)">
    Source: Source Name (Year)
  </text>
</svg>

JSX/MDX Shell (camelCase attributes)

<svg
  viewBox="0 0 560 380"
  style={{maxWidth: '100%', height: 'auto', fontFamily: "'Inter', system-ui, sans-serif"}}
  role="img"
  aria-labelledby="chart-title chart-desc"
>
  <title id="chart-title">Chart Title</title>
  <desc id="chart-desc">Description for screen readers</desc>

  {/* Chart content */}

  <text x="280" y="372" textAnchor="middle" fontSize="10" fill="var(--chart-muted, currentColor)">
    Source: Source Name (Year)
  </text>
</svg>

JSX Attribute Conversion (Required for MDX)

Chart Type Construction

Horizontal Bar Chart

Best for: percentage improvements, single-metric comparisons.

1. Define chart area: x=80, y=40, width=440, height=280
2. Calculate bar height: chartHeight / dataCount - gap (gap=8)
3. Calculate bar width: (value / maxValue) * chartWidth
4. Position bars: y = chartY + index * (barHeight + gap)
5. Label on left (right-aligned at x=75): category name
6. Value label at end of bar: percentage or number
7. Source text at bottom center

Grouped Bar Chart

Best for: before/after, A vs B comparisons.

1. Define groups along Y axis, bars within each group
2. Use 2 colors (primary + secondary) for the two series
3. Add legend at top: colored square + label for each series
4. Gap between groups > gap within groups

Donut Chart

Best for: parts of whole, market share.

1. Center: cx=280, cy=180, outer radius=140, inner radius=80
2. Calculate arc segments using cumulative angles
3. Each segment: <path d="M... A... L... A... Z" fill="color" />
4. Center text: total or key label
5. Legend below chart with color squares + labels + values

Line Chart

Best for: trends over time.

1. X axis: time periods, evenly spaced
2. Y axis: value range with 4-5 grid lines
3. Draw grid lines: stroke="currentColor" opacity="0.08"
4. Plot data points: <circle cx=... cy=... r="4" fill="color" />
5. Connect with: <polyline points="..." fill="none" stroke="color" strokeWidth="2" />
6. Optional: area fill below line with opacity="0.1"

Lollipop Chart

Best for: ranked factors, correlations.

1. Horizontal orientation (like bar chart but with circles)
2. Thin line from axis to data point: stroke="currentColor" opacity="0.15" strokeWidth="1"
3. Circle at data point: r="6" with fill color
4. Value label next to circle
5. Categories on Y axis (left-aligned)

Area Chart

Best for: distribution, cumulative data.

1. Same as line chart but with filled area below
2. Area fill: <path d="M... L... L... Z" fill="color" opacity="0.15" />
3. Line on top: stroke="color" strokeWidth="2" fill="none"
4. Grid lines behind the area

Radar Chart

Best for: multi-dimensional scoring (5-7 axes).

1. Center: cx=280, cy=190
2. Draw concentric polygons for grid (3-4 levels)
3. Calculate axis endpoints at equal angles
4. Plot data points on each axis proportional to value
5. Connect data points with filled polygon: fill="color" opacity="0.2" stroke="color"
6. Label each axis at the outer edge

Label Rules

• Wrap long labels at word boundaries into <tspan> lines.
• Truncate only when wrapping would collide with data marks, and keep the full label in <desc> or adjacent prose.
• Use stable chart dimensions with a responsive max-width: 100%; height: auto style, or choose a justified wider viewBox for dense labels.
• Check mobile widths so axis labels, legends, and value labels do not overlap.

Output Format

Wrap every chart in a <figure> element:

HTML:

<figure>
  <svg viewBox="0 0 560 380" style="max-width: 100%; height: auto; font-family: 'Inter', system-ui, sans-serif" role="img" aria-labelledby="chart-title chart-desc">
    <title id="chart-title">[Chart Title]</title>
    <desc id="chart-desc">[Full description with data points for screen readers]</desc>
    <!-- chart content -->
    <text x="280" y="372" text-anchor="middle" font-size="10" fill="var(--chart-muted, currentColor)">
      Source: [Source Name] ([Year])
    </text>
  </svg>
  <figcaption>Source: <a href="[Source URL]">[Source Name]</a>, [publication date].</figcaption>
</figure>

MDX:

<figure className="chart-container" style={{margin: '2.5rem 0', textAlign: 'center', padding: '1.5rem', borderRadius: '12px'}}>
  <svg viewBox="0 0 560 380" style={{maxWidth: '100%', height: 'auto', fontFamily: "'Inter', system-ui, sans-serif"}} role="img" aria-labelledby="chart-title chart-desc">
    <title id="chart-title">[Chart Title]</title>
    <desc id="chart-desc">[Full description]</desc>
    {/* chart content with camelCase attributes */}
    <text x="280" y="372" textAnchor="middle" fontSize="10" fill="var(--chart-muted, currentColor)">
      Source: [Source Name] ([Year])
    </text>
  </svg>
  <figcaption>Source: <a href="[Source URL]">[Source Name]</a>, [publication date].</figcaption>
</figure>

Quality Checklist (Verify Before Returning)

• No hardcoded text colors except contrast-checked labels inside colored elements
• No white/light backgrounds (transparent or none)
• Source attribution text present at bottom and semantic <figcaption> present
• role="img" and aria-labelledby present on <svg>
• <title id> and <desc id> present inside <svg>
• Chart type choice supports comprehension and comparability
• If MDX: all attributes camelCased (no hyphens in attribute names)
• Data values match the source data exactly
• Color palette uses only approved colors
• ViewBox is 0 0 560 380 (standard) or justified alternative
• Labels, shapes, patterns, or line styles provide redundancy beyond color