charting-vega-lite

## Overview

This skill creates interactive Vega-Lite visualizations from uploaded data. The workflow:
1. Analyze data structure and context
2. Select 5-10 meaningful chart types based on what the data represents
3. Build chart specifications programmatically
4. Generate React artifact with embedded visualizations

## Critical Technical Constraint: Inline Data Island

**Claude artifacts cannot use fetch() for computer:// URLs.**

All data must be embedded as an inline JavaScript constant:

```javascript
const DATA = [ /* embedded data array */ ];

// Later in chart specs:
spec.data = { values: DATA };
```

**DO NOT:**
- Use fetch() to load external files
- Reference external data URLs
- Create separate data files

This is the only pattern that works in Claude's artifact environment.

## Primary Workflow: Data Upload → Chart Explorer

Execute this sequence when user uploads data without specifying chart type:

### 1. Analyze Data Structure

```bash
python /mnt/skills/user/charting-vega-lite/scripts/analyze_data.py /mnt/user-data/uploads/<filename>
```

**Extract from output:** 
- `fields[]` (with types and statistics)
- `suggested_charts[]` (suggested chart types with encodings)
- `sample_data` (first 10 rows for understanding context)

**If script fails:** Use manual pandas analysis
```python
import pandas as pd
df = pd.read_csv('/mnt/user-data/uploads/<filename>')
# Classify: numeric→quantitative, datetime→temporal, <20 unique→nominal
```

### 2. Understand Data Context

**Read sample data and column names to infer what the data represents:**

- **Biomedical data?** → Biomarkers, patient outcomes, clinical relevance
- **Financial data?** → Trends, comparisons, performance metrics
- **Sensor data?** → Temporal patterns, anomalies, correlations
- **E-commerce?** → Sales trends, product comparisons, conversions

**Ask:** What questions would someone analyzing this data want answered?

Examples:
- Assay data: Which biomarkers strongest? Patterns across samples? Variability?
- Financial: What are trends? How volatile? Seasonal patterns?
- IoT: Temporal patterns? Anomalies? Sensor correlations?

### 3. Select Meaningful Charts (5-10 suggestions)

**Filter analyze_data.py suggestions based on context and readability:**

**Apply readability filters:**
- Pie chart with >7 categories → Skip (unreadable)
- Heatmap with >50 categories per axis → Aggregate first
- Multi-line with >10 series → Consider faceting

**Prioritize charts that answer domain questions:**
- Comparison needs → Bar, box plot, grouped bar
- Distribution analysis → Histogram, box plot
- Pattern recognition → Heatmap, scatter
- Temporal trends → Line, area
- Part-to-whole → Stacked bar (pie only if <7 categories)

**Don't suggest charts just because data types match - choose charts that reveal insights.**

### 4. Generate Chart Specs

**Build specs programmatically using analyze_data.py encodings:**

For each suggested chart type, construct spec using:
- Templates from `assets/templates/` for basic types (bar, line, scatter, pie, heatmap, area)
- Builder patterns from `references/spec-builder-patterns.md` for variations (histogram, boxplot, grouped-bar, etc.)
- Vega-Lite examples from `references/vega-lite-examples-inventory.md` for uncommon types

Structure each chart as:
```python
{"type": "Chart Name", "reason": "Why this chart", "spec": {/* vega-lite spec */}}
```

### 5. Create Artifact with Inline Data Island

Load data, read template, replace `__DATA__` and `__CHART_SPECS__` placeholders, write using bash heredoc.

### 6. Provide Link

```
[View chart explorer](computer:///mnt/user-data/outputs/ChartExplorer.jsx)

Created 7 contextually relevant charts for your data.
```

## Secondary Workflow: Specific Chart Request

When user specifies chart type (e.g., "make a bar chart"):

### 1. Analyze Data
```bash
python /mnt/skills/user/charting-vega-lite/scripts/analyze_data.py /mnt/user-data/uploads/<filename>
```

### 2. Validate Chart Fits Data

**Check requirements:**
- Bar: needs 1 nominal + 1 quantitative
- Line: needs 1 temporal + 1 quantitative
- Scatter: needs 2 quantitative
- Heatmap: needs 2 nominal + 1 quantitative
- Pie: needs 1 nominal + 1 quantitative + <7 categories

**If data doesn't fit:**
- Explain: "Bar chart needs categorical data, but all columns are numeric"
- Suggest 2-3 alternatives
- Use Primary Workflow to create explorer with alternatives

### 3. Generate Spec

Use templates or programmatic builders based on chart type complexity.

### 4. Create Artifact

Same pattern as Primary Workflow step 5, but with single chart.

## Error Prevention

**Common failures:**

1. **Using fetch() in artifacts**
   - Solution: Always use inline data island pattern
   - Never create external data files

2. **Chart doesn't render**
   - Verify scripts load: Vega → Vega-Lite → Vega-Embed
   - Check data is injected: `spec.data = {values: DATA}`
   - Confirm field names match data columns

3. **Generic/random chart suggestions**
   - Solution: Consider data context and meaning
   - Filter suggestions for relevance and readability
   - Prioritize charts that answer meaningful questions

## Resources

**Scripts:**
- `scripts/analyze_data.py` - analyze structure, suggest 8-12 chart types

**Components:**
- `assets/components/ChartExplorer.jsx` - multi-chart explorer template

**Templates:**
- `assets/templates/*.json` - 6 basic chart templates (bar, line, scatter, pie, heatmap, area)

**References - Progressive Disclosure:**

Read `spec-builder-patterns.md` when building charts programmatically (histogram, boxplot, grouped/stacked bars, multi-line, etc.)

Read `vega-lite-examples-inventory.md` when user requests uncommon chart type not in spec-builder-patterns

Read `chart-types.md` when validating specific chart requirements or user asks "what chart should I use for..."

Read `advanced-charts.md` for complete specs of specialized charts (sankey, waterfall, violin plots, complex layered compositions)

Read `contextual-chart-sel