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)

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

Chart Types

Line Chart (default)

node chart.mjs --type line --data '[{"x":"A","y":10},{"x":"B","y":15}]' --output line.png

Bar Chart

node chart.mjs --type bar --data '[{"x":"A","y":10},{"x":"B","y":15}]' --output bar.png

Area Chart

node chart.mjs --type area --data '[{"x":"A","y":10},{"x":"B","y":15}]' --output area.png

Pie / Donut Chart

# Pie
node chart.mjs --type pie --data '[{"category":"A","value":30},{"category":"B","value":70}]' \
  --category-field category --y-field value --output pie.png

# Donut (with hole)
node chart.mjs --type donut --data '[{"category":"A","value":30},{"category":"B","value":70}]' \
  --category-field category --y-field value --output donut.png

Candlestick Chart (OHLC)

node chart.mjs --type candlestick \
  --data '[{"x":"Mon","open":100,"high":110,"low":95,"close":105}]' \
  --open-field open --high-field high --low-field low --close-field close \
  --title "Stock Price" --output candle.png

Heatmap

node chart.mjs --type heatmap \
  --data '[{"x":"Mon","y":"Week1","value":5},{"x":"Tue","y":"Week1","value":8}]' \
  --color-value-field value --color-scheme viridis \
  --title "Activity Heatmap" --output heatmap.png

Multi-Series Line Chart

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"}]' \
  --title "Comparison" --output multi.png

Stacked Bar Chart

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

Volume Overlay (Dual Y-axis)

Price line with volume bars:

node chart.mjs --type line --volume-field volume \
  --data '[{"x":"10:00","y":100,"volume":5000},{"x":"11:00","y":105,"volume":3000}]' \
  --title "Price + Volume" --output volume.png

Sparkline (mini inline chart)

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

Sparklines are 80x20 by default, transparent, no axes.

Options Reference

Basic Options

Option	Description	Default
--type	Chart type: line, bar, area, point, pie, donut, candlestick, heatmap	line
--data	JSON array of data points	-
--output	Output file path	chart.png
--title	Chart title	-
--width	Width in pixels	600
--height	Height in pixels	300

Axis Options

Option	Description	Default
--x-field	Field name for X axis	x
--y-field	Field name for Y axis	y
--x-title	X axis label	field name
--y-title	Y axis label	field name
--x-type	X axis type: ordinal, temporal, quantitative	ordinal
--y-domain	Y scale as "min,max"	auto

Visual Options

Option	Description	Default
--color	Line/bar color	#e63946
--dark	Dark mode theme	false
--svg	Output SVG instead of PNG	false
--color-scheme	Vega color scheme (category10, viridis, etc.)	-

Alert/Monitor Options

Option	Description	Default
--show-change	Show +/-% change annotation at last point	false
--focus-change	Zoom Y-axis to 2x data range	false
--focus-recent N	Show only last N data points	all
--show-values	Label min/max peak points	false

Multi-Series/Stacked Options

Option	Description	Default
--series-field	Field for multi-series line charts	-
--stacked	Enable stacked bar mode	false
--color-field	Field for stack/color categories	-

Candlestick Options

Option	Description	Default
--open-field	OHLC open field	open
--high-field	OHLC high field	high
--low-field	OHLC low field	low
--close-field	OHLC close field	close

Pie/Donut Options

Option	Description	Default
--category-field	Field for pie slice categories	x
--donut	Render as donut (with center hole)	false

Heatmap Options

Option	Description	Default
--color-value-field	Field for heatmap intensity	value
--y-category-field	Y axis category field	y

Dual-Axis Options (General)

Option	Description	Default
--y2-field	Second Y axis field (independent right axis)	-
--y2-title	Title for second Y axis	field name
--y2-color	Color for second series	#60a5fa (dark) / #2563eb (light)
--y2-type	Chart type for second axis: line, bar, area	line

Example: Revenue bars (left) + Churn area (right):

node chart.mjs \
  --data '[{"month":"Jan","revenue":12000,"churn":4.2},...]' \
  --x-field month --y-field revenue --type bar \
  --y2-field churn --y2-type area --y2-color "#60a5fa" \
  --y-title "Revenue ($)" --y2-title "Churn (%)" \
  --x-sort none --dark --title "Revenue vs Churn"

Volume Overlay Options (Candlestick)

Option	Description	Default
--volume-field	Field for volume bars (enables dual-axis)	-
--volume-color	Color for volume bars	#4a5568

Formatting Options

Option	Description	Default
--y-format	Y axis format: percent, dollar, compact, decimal4, integer, scientific, or d3-format string	auto
--subtitle	Subtitle text below chart title	-
--hline	Horizontal reference line: "value" or "value,color" or "value,color,label" (repeatable)	-

Annotation Options

Option	Description	Default
--annotation	Static text annotation	-
--annotations	JSON array of event markers	-

Alert-Style Chart (recommended for monitors)

node chart.mjs --type line --data '[...]' \
  --title "Iran Strike Odds (48h)" \
  --show-change --focus-change --show-values --dark \
  --output alert.png

For recent action only:

node chart.mjs --type line --data '[hourly data...]' \
  --focus-recent 4 --show-change --focus-change --dark \
  --output recent.png

Timeline Annotations

Mark events on the chart:

node chart.mjs --type line --data '[...]' \
  --annotations '[{"x":"14:00","label":"News broke"},{"x":"16:30","label":"Press conf"}]' \
  --output annotated.png

Temporal X-Axis

For proper time series with date gaps:

node chart.mjs --type line --x-type temporal \
  --data '[{"x":"2026-01-01","y":10},{"x":"2026-01-15","y":20}]' \
  --output temporal.png

Use --x-type temporal when X values are ISO dates and you want spacing to reflect actual time gaps (not evenly spaced).

Y-Axis Formatting

Format axis values for readability:

# Dollar amounts
node chart.mjs --data '[...]' --y-format dollar --output revenue.png
# → $1,234.56

# Percentages (values as decimals 0-1)
node chart.mjs --data '[...]' --y-format percent --output rates.png
# → 45.2%

# Compact large numbers
node chart.mjs --data '[...]' --y-format compact --output users.png
# → 1.2K, 3.4M

# Crypto prices (4 decimal places)
node chart.mjs --data '[...]' --y-format decimal4 --output molt.png
# → 0.0004

# Custom d3-format string
node chart.mjs --data '[...]' --y-format ',.3f' --output custom.png

Available shortcuts: percent, dollar/usd, compact, integer, decimal2, decimal4, scientific

Chart Subtitle

Add context below the title:

node chart.mjs --title "MOLT Price" --subtitle "20,668 MOLT held" --data '[...]' --output molt.png

Theme Selection

Use --dark for dark mode. Auto-select based on time:

• Night (20:00-07:00 local): --dark
• Day (07:00-20:00 local): light mode (default)

Piping Data

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

Custom Vega-Lite Spec

For advanced charts:

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

⚠️ IMPORTANT: Always Send the Image!

After generating a chart, always send it back to the user's channel.
Don't just save to a file and describe it — the whole point is the visual.

# 1. Generate the chart
node chart.mjs --type line --data '...' --output /data/clawd/tmp/my-chart.png

# 2. Send it! Use message tool with filePath:
#    action=send, target=<channel_id>, filePath=/data/clawd/tmp/my-chart.png

Tips:

• Save to /data/clawd/tmp/ (persistent) not /tmp/ (may get cleaned)
• Use action=send with filePath — thread-reply does NOT support file attachments
• Include a brief caption in the message text
• Auto-use --dark between 20:00-07:00 Israel time

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Updated: 2026-02-04 - Added --y-format (percent/dollar/compact/decimal4) and --subtitle