---
name: Chart Image
slug: chart-image-2
category: Data
description: Chart Image generates PNG chart images from data, supporting line, bar, area, candlestick, pie, and heatmap charts. Use it when you need to visualize data, plot time series, or create charts for reports, alerts, or dashboards without a browser.
github: "https://github.com/zebbern/claude-code-guide/tree/main/skills/chart-image"
language: Python
stars: 4571
forks: 459
install: "npx degit https://github.com/zebbern/claude-code-guide/tree/main/skills/chart-image ~/.claude/skills/chart-image"
installs_to: ~/.claude/skills/chart-image
source_path: skills/chart-image/SKILL.md
collection_size: 25
category_size: 668
collection_url: "https://dirskills.com/collections/zebbern/claude-code-guide"
added: 2026-08-16T07:01:13.343Z
last_synced: 2026-08-16T07:01:13.343Z
canonical_url: "https://dirskills.com/skills/chart-image-2"
---

# Chart Image

Chart Image generates PNG chart images from data, supporting line, bar, area, candlestick, pie, and heatmap charts. Use it when you need to visualize data, plot time series, or create charts for reports, alerts, or dashboards without a browser.

**Install:**

```bash
npx degit https://github.com/zebbern/claude-code-guide/tree/main/skills/chart-image ~/.claude/skills/chart-image
```

## README

# 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)

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

## Quick Usage

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

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

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

### Pie / Donut Chart
```bash
# 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)
```bash
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
```bash
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:
```bash
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
```bash
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:
```bash
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)
```bash
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):
```bash
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)

```bash
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:
```bash
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:
```bash
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:
```bash
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:
```bash
# 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:
```bash
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

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

## Custom Vega-Lite Spec

For advanced charts:
```bash
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.

```bash
# 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

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