d3-viz

What this skill does

# D3.js Visualisation

## Overview

This skill provides guidance for creating sophisticated, interactive data visualisations using d3.js. D3.js (Data-Driven Documents) excels at binding data to DOM elements and applying data-driven transformations to create custom, publication-quality visualisations with precise control over every visual element. The techniques work across any JavaScript environment, including vanilla JavaScript, React, Vue, Svelte, and other frameworks.

## When to use d3.js

**Use d3.js for:**
- Custom visualisations requiring unique visual encodings or layouts
- Interactive explorations with complex pan, zoom, or brush behaviours
- Network/graph visualisations (force-directed layouts, tree diagrams, hierarchies, chord diagrams)
- Geographic visualisations with custom projections
- Visualisations requiring smooth, choreographed transitions
- Publication-quality graphics with fine-grained styling control
- Novel chart types not available in standard libraries

**Consider alternatives for:**
- 3D visualisations - use Three.js instead

## Core workflow

### 1. Set up d3.js

Import d3 at the top of your script:

```javascript
import * as d3 from 'd3';
```

Or use the CDN version (7.x):

```html
<script src="https://d3js.org/d3.v7.min.js"></script>
```

All modules (scales, axes, shapes, transitions, etc.) are accessible through the `d3` namespace.

### 2. Choose the integration pattern

**Pattern A: Direct DOM manipulation (recommended for most cases)**
Use d3 to select DOM elements and manipulate them imperatively. This works in any JavaScript environment:

```javascript
function drawChart(data) {
  if (!data || data.length === 0) return;

  const svg = d3.select('#chart'); // Select by ID, class, or DOM element

  // Clear previous content
  svg.selectAll("*").remove();

  // Set up dimensions
  const width = 800;
  const height = 400;
  const margin = { top: 20, right: 30, bottom: 40, left: 50 };

  // Create scales, axes, and draw visualisation
  // ... d3 code here ...
}

// Call when data changes
drawChart(myData);
```

**Pattern B: Declarative rendering (for frameworks with templating)**
Use d3 for data calculations (scales, layouts) but render elements via your framework:

```javascript
function getChartElements(data) {
  const xScale = d3.scaleLinear()
    .domain([0, d3.max(data, d => d.value)])
    .range([0, 400]);

  return data.map((d, i) => ({
    x: 50,
    y: i * 30,
    width: xScale(d.value),
    height: 25
  }));
}

// In React: {getChartElements(data).map((d, i) => <rect key={i} {...d} fill="steelblue" />)}
// In Vue: v-for directive over the returned array
// In vanilla JS: Create elements manually from the returned data
```

Use Pattern A for complex visualisations with transitions, interactions, or when leveraging d3's full capabilities. Use Pattern B for simpler visualisations or when your framework prefers declarative rendering.

### 3. Structure the visualisation code

Follow this standard structure in your drawing function:

```javascript
function drawVisualization(data) {
  if (!data || data.length === 0) return;

  const svg = d3.select('#chart'); // Or pass a selector/element
  svg.selectAll("*").remove(); // Clear previous render

  // 1. Define dimensions
  const width = 800;
  const height = 400;
  const margin = { top: 20, right: 30, bottom: 40, left: 50 };
  const innerWidth = width - margin.left - margin.right;
  const innerHeight = height - margin.top - margin.bottom;

  // 2. Create main group with margins
  const g = svg.append("g")
    .attr("transform", `translate(${margin.left},${margin.top})`);

  // 3. Create scales
  const xScale = d3.scaleLinear()
    .domain([0, d3.max(data, d => d.x)])
    .range([0, innerWidth]);

  const yScale = d3.scaleLinear()
    .domain([0, d3.max(data, d => d.y)])
    .range([innerHeight, 0]); // Note: inverted for SVG coordinates

  // 4. Create and append axes
  const xAxis = d3.axisBottom(xScale);
  const yAxis = d3.axisLeft(yScale);

  g.append("g")
    .attr("transform", `translate(0,${innerHeight})`)
    .call(xAxis);

  g.append("g")
    .call(yAxis);

  // 5. Bind data and create visual elements
  g.selectAll("circle")
    .data(data)
    .join("circle")
    .attr("cx", d => xScale(d.x))
    .attr("cy", d => yScale(d.y))
    .attr("r", 5)
    .attr("fill", "steelblue");
}

// Call when data changes
drawVisualization(myData);
```

### 4. Implement responsive sizing

Make visualisations responsive to container size:

```javascript
function setupResponsiveChart(containerId, data) {
  const container = document.getElementById(containerId);
  const svg = d3.select(`#${containerId}`).append('svg');

  function updateChart() {
    const { width, height } = container.getBoundingClientRect();
    svg.attr('width', width).attr('height', height);

    // Redraw visualisation with new dimensions
    drawChart(data, svg, width, height);
  }

  // Update on initial load
  updateChart();

  // Update on window resize
  window.addEventListener('resize', updateChart);

  // Return cleanup function
  return () => window.removeEventListener('resize', updateChart);
}

// Usage:
// const cleanup = setupResponsiveChart('chart-container', myData);
// cleanup(); // Call when component unmounts or element removed
```

Or use ResizeObserver for more direct container monitoring:

```javascript
function setupResponsiveChartWithObserver(svgElement, data) {
  const observer = new ResizeObserver(() => {
    const { width, height } = svgElement.getBoundingClientRect();
    d3.select(svgElement)
      .attr('width', width)
      .attr('height', height);

    // Redraw visualisation
    drawChart(data, d3.select(svgElement), width, height);
  });

  observer.observe(svgElement.parentElement);
  return () => observer.disconnect();
}
```

## Common visualisation patterns

### Bar chart

```javascript
function drawBarChart(data, svgElement) {
  if (!data || data.length === 0) return;

  const svg = d3.select(svgElement);
  svg.selectAll("*").remove();

  const width = 800;
  const height = 400;
  const margin = { top: 20, right: 30, bottom: 40, left: 50 };
  const innerWidth = width - margin.left - margin.right;
  const innerHeight = height - margin.top - margin.bottom;

  const g = svg.append("g")
    .attr("transform", `translate(${margin.left},${margin.top})`);

  const xScale = d3.scaleBand()
    .domain(data.map(d => d.category))
    .range([0, innerWidth])
    .padding(0.1);

  const yScale = d3.scaleLinear()
    .domain([0, d3.max(data, d => d.value)])
    .range([innerHeight, 0]);

  g.append("g")
    .attr("transform", `translate(0,${innerHeight})`)
    .call(d3.axisBottom(xScale));

  g.append("g")
    .call(d3.axisLeft(yScale));

  g.selectAll("rect")
    .data(data)
    .join("rect")
    .attr("x", d => xScale(d.category))
    .attr("y", d => yScale(d.value))
    .attr("width", xScale.bandwidth())
    .attr("height", d => innerHeight - yScale(d.value))
    .attr("fill", "steelblue");
}

// Usage:
// drawBarChart(myData, document.getElementById('chart'));
```

### Line chart

```javascript
const line = d3.line()
  .x(d => xScale(d.date))
  .y(d => yScale(d.value))
  .curve(d3.curveMonotoneX); // Smooth curve

g.append("path")
  .datum(data)
  .attr("fill", "none")
  .attr("stroke", "steelblue")
  .attr("stroke-width", 2)
  .attr("d", line);
```

### Scatter plot

```javascript
g.selectAll("circle")
  .data(data)
  .join("circle")
  .attr("cx", d => xScale(d.x))
  .attr("cy", d => yScale(d.y))
  .attr("r", d => sizeScale(d.size)) // Optional: size encoding
  .attr("fill", d => colourScale(d.category)) // Optional: colour encoding
  .attr("opacity", 0.7);
```

### Chord diagram

A chord diagram shows relationships between entities in a circular layout, with ribbons representing flows between them:

```javascript
function drawChordDiagram(data) {
  // data format: array of objects with source, target, and value
  // Example: [{ source: 'A', target: 'B', value: 10 }, ...]

  if (!d