Chart Image Generator

---

name: chart-image

version: 2.6.14

description: Generate publication-quality chart images from data. Supports line, bar, area, point, candlestick, pie/donut, heatmap, multi-series, and stacked charts. Use when visualizing data, creating graphs, plotting time series, or generating chart images for reports/alerts. Designed for Fly.io/VPS deployments - no native compilation, no Puppeteer, no browser required. Pure Node.js with prebuilt binaries.

provides:

- capability: chart-generation

methods: [lineChart, barChart, areaChart, pieChart, candlestickChart, heatmap]

---

# 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:**

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 |

| `--x-sort` | X axis order: ascending, descending, or none (preserve input order) | auto |

| `--y-domain` | Y scale as "min,max" | auto |

| `--y-pad` | Add vertical padding as a fraction of range (e.g. `0.1` = 10%) | 0 |

Visual Options

| Option | Description | Default |

|--------|-------------|---------|

| `--color` | Line/bar color | #e63946 |

| `--dark` | Dark mode theme | false |

| `--svg` | Output SVG instead of PNG | false |

| `--no-points` | Hide point markers on line charts | false |

| `--line-width N` | Set line thickness in pixels for line charts | 2 |

| `--point-size N` | Set point marker size for line/point charts | 60 |

| `--bar-radius N` | Round bar corners in pixels for bar-based charts | 0 |

| `--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 |

| `--last-value` | Label the final data point value | 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:

Social Size Presets

Use `--output-size` when the chart is meant for a specific surface:

# Bluesky / OG-style landscape post
node chart.mjs --type line --data '[...]' --output-size bluesky --output bluesky-chart.png

# Instagram / Threads portrait post
node chart.mjs --type line --data '[...]' --output-size portrait --output portrait-chart.png

Available presets include `twitter`, `discord`, `slack`, `linkedin`, `bluesky` (`bsky` alias), `youtube`, `instagram`, `portrait`, `story`, `thumbnail`, `wide`, and `square`.

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:**

---

*Updated: 2026-03-17 - Added `--bar-radius` for rounded corners on bar-based charts (including stacked/volume/double-axis bar uses); version bumped to 2.6.14*