Chart Image Generator

Generate PNG chart images from data using Vega-Lite. Perfect for headless server environments.

Why This Skill?

Built for Fly.io / VPS / Docker deployments:

• ✅ No native compilation - Uses Sharp with prebuilt binaries (unlike canvas which requires build tools)
• ✅ No Puppeteer/browser - Pure Node.js, no Chrome download, no headless browser overhead
• ✅ Lightweight - ~15MB total dependencies vs 400MB+ for Puppeteer-based solutions
• ✅ Fast cold starts - No browser spinup delay, generates charts in <500ms
• ✅ Works offline - No external API calls (unlike QuickChart.io)

Alternatives and why they don't work well on Fly:

• canvas npm - Requires native compilation, fails on minimal Docker images
• Puppeteer + Chart.js - 400MB+ Chrome download, slow cold starts, memory heavy
• QuickChart API - External dependency, rate limits, privacy concerns
• Plotly - Needs Puppeteer for static image export

Setup (one-time)

cd /data/clawd/skills/chart-image/scripts && npm install

Quick Usage

node /data/clawd/skills/chart-image/scripts/chart.mjs \
  --type line \
  --data '[{"x":"10:00","y":25},{"x":"10:30","y":27},{"x":"11:00","y":31}]' \
  --title "Price Over Time" \
  --output chart.png

Alert-Style Chart (recommended for monitors)

# Full data with zoomed Y-axis
node /data/clawd/skills/chart-image/scripts/chart.mjs \
  --type line \
  --data '[...]' \
  --title "Elon Posts 34 Tweets This Week" \
  --show-change \
  --focus-change \
  --show-values \
  --dark \
  --output alert.png

# Focus on recent action (last 4 points)
node /data/clawd/skills/chart-image/scripts/chart.mjs \
  --type line \
  --data '[hourly data...]' \
  --title "Elon Tweet Odds (24h)" \
  --show-change \
  --focus-change \
  --focus-recent 4 \
  --show-values \
  --dark \
  --output alert-recent.png

Options

Multi-Series Line Charts

Compare multiple trends on one chart:

node chart.mjs \
  --type line \
  --series-field "market" \
  --data '[{"x":"Jan","y":10,"market":"A"},{"x":"Jan","y":15,"market":"B"},{"x":"Feb","y":12,"market":"A"},{"x":"Feb","y":18,"market":"B"}]' \
  --title "Comparison" \
  --output multi.png

Each unique value in --series-field becomes a separate colored line with legend.

Stacked Bar Charts

Visualize breakdowns by category:

node chart.mjs \
  --type bar \
  --stacked \
  --color-field "category" \
  --data '[{"x":"Mon","y":10,"category":"Work"},{"x":"Mon","y":5,"category":"Personal"}]' \
  --title "Hours by Category" \
  --output stacked.png

Data needs three fields: x (groups), y (values), and the color-field (stack categories).

Sparklines (inline mini-charts)

Tiny charts for embedding in text summaries:

node chart.mjs \
  --sparkline \
  --data '[{"x":"1","y":10},{"x":"2","y":15},{"x":"3","y":12}]' \
  --output spark.png

Sparklines are 80x20 by default (override with --width / --height), transparent background, no axes.

Theme Selection

Use --dark for dark mode. Recommended to auto-select based on time:

• Night (20:00-07:00 local): --dark
• Day (07:00-20:00 local): no flag (light mode)

Piping Data

echo '[{"x":"A","y":1},{"x":"B","y":2}]' | node chart.mjs --output out.png

Custom Vega-Lite Spec

For advanced charts, pass a full Vega-Lite spec:

node chart.mjs --spec my-spec.json --output custom.png