---
name: d3-viz
description: Creating interactive data visualisations using d3.js. This skill should be used when creating custom charts, graphs, network diagrams, geographic visualisations, or any complex SVG-based data visualisation that requires fine-grained control over visual elements, transitions, or interactions. Use this for bespoke visualisations beyond standard charting libraries, whether in React, Vue, Svelte, vanilla JavaScript, or any other environment.
---

# 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 (!data || data.length === 0) return;

  const svg = d3.select('#chart');
  svg.selectAll("*").remove();

  const width = 600;
  const height = 600;
  const innerRadius = Math.min(width, height) * 0.3;
  const outerRadius = innerRadius + 30;

  // Create matrix from data
  const nodes = Array.from(new Set(data.flatMap(d => [d.source, d.target])));
  const matrix = Array.from({ length: nodes.length }, () => Array(nodes.length).fill(0));

  data.forEach(d => {
    const i = nodes.indexOf(d.source);
    const j = nodes.indexOf(d.target);
    matrix[i][j] += d.value;
    matrix[j][i] += d.value;
  });

  // Create chord layout
  const chord = d3.chord()
    .padAngle(0.05)
    .sortSubgroups(d3.descending);

  const arc = d3.arc()
    .innerRadius(innerRadius)
    .outerRadius(outerRadius);

  const ribbon = d3.ribbon()
    .source(d => d.source)
    .target(d => d.target);

  const colourScale = d3.scaleOrdinal(d3.schemeCategory10)
    .domain(nodes);

  const g = svg.append("g")
    .attr("transform", `translate(${width / 2},${height / 2})`);

  const chords = chord(matrix);

  // Draw ribbons
  g.append("g")
    .attr("fill-opacity", 0.67)
    .selectAll("path")
    .data(chords)
    .join("path")
    .attr("d", ribbon)
    .attr("fill", d => colourScale(nodes[d.source.index]))
    .attr("stroke", d => d3.rgb(colourScale(nodes[d.source.index])).darker());

  // Draw groups (arcs)
  const group = g.append("g")
    .selectAll("g")
    .data(chords.groups)
    .join("g");

  group.append("path")
    .attr("d", arc)
    .attr("fill", d => colourScale(nodes[d.index]))
    .attr("stroke", d => d3.rgb(colourScale(nodes[d.index])).darker());

  // Add labels
  group.append("text")
    .each(d => { d.angle = (d.startAngle + d.endAngle) / 2; })
    .attr("dy", "0.31em")
    .attr("transform", d => `rotate(${(d.angle * 180 / Math.PI) - 90})translate(${outerRadius + 30})${d.angle > Math.PI ? "rotate(180)" : ""}`)
    .attr("text-anchor", d => d.angle > Math.PI ? "end" : null)
    .text((d, i) => nodes[i])
    .style("font-size", "12px");
}
```

### Heatmap

A heatmap uses colour to encode values in a two-dimensional grid, useful for showing patterns across categories:

```javascript
function drawHeatmap(data) {
  // data format: array of objects with row, column, and value
  // Example: [{ row: 'A', column: 'X', value: 10 }, ...]

  if (!data || data.length === 0) return;

  const svg = d3.select('#chart');
  svg.selectAll("*").remove();

  const width = 800;
  const height = 600;
  const margin = { top: 100, right: 30, bottom: 30, left: 100 };
  const innerWidth = width - margin.left - margin.right;
  const innerHeight = height - margin.top - margin.bottom;

  // Get unique rows and columns
  const rows = Array.from(new Set(data.map(d => d.row)));
  const columns = Array.from(new Set(data.map(d => d.column)));

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

  // Create scales
  const xScale = d3.scaleBand()
    .domain(columns)
    .range([0, innerWidth])
    .padding(0.01);

  const yScale = d3.scaleBand()
    .domain(rows)
    .range([0, innerHeight])
    .padding(0.01);

  // Colour scale for values
  const colourScale = d3.scaleSequential(d3.interpolateYlOrRd)
    .domain([0, d3.max(data, d => d.value)]);

  // Draw rectangles
  g.selectAll("rect")
    .data(data)
    .join("rect")
    .attr("x", d => xScale(d.column))
    .attr("y", d => yScale(d.row))
    .attr("width", xScale.bandwidth())
    .attr("height", yScale.bandwidth())
    .attr("fill", d => colourScale(d.value));

  // Add x-axis labels
  svg.append("g")
    .attr("transform", `translate(${margin.left},${margin.top})`)
    .selectAll("text")
    .data(columns)
    .join("text")
    .attr("x", d => xScale(d) + xScale.bandwidth() / 2)
    .attr("y", -10)
    .attr("text-anchor", "middle")
    .text(d => d)
    .style("font-size", "12px");

  // Add y-axis labels
  svg.append("g")
    .attr("transform", `translate(${margin.left},${margin.top})`)
    .selectAll("text")
    .data(rows)
    .join("text")
    .attr("x", -10)
    .attr("y", d => yScale(d) + yScale.bandwidth() / 2)
    .attr("dy", "0.35em")
    .attr("text-anchor", "end")
    .text(d => d)
    .style("font-size", "12px");

  // Add colour legend
  const legendWidth = 20;
  const legendHeight = 200;
  const legend = svg.append("g")
    .attr("transform", `translate(${width - 60},${margin.top})`);

  const legendScale = d3.scaleLinear()
    .domain(colourScale.domain())
    .range([legendHeight, 0]);

  const legendAxis = d3.axisRight(legendScale)
    .ticks(5);

  // Draw colour gradient in legend
  for (let i = 0; i < legendHeight; i++) {
    legend.append("rect")
      .attr("y", i)
      .attr("width", legendWidth)
      .attr("height", 1)
      .attr("fill", colourScale(legendScale.invert(i)));
  }

  legend.append("g")
    .attr("transform", `translate(${legendWidth},0)`)
    .call(legendAxis);
}
```

### Pie chart

```javascript
const pie = d3.pie()
  .value(d => d.value)
  .sort(null);

const arc = d3.arc()
  .innerRadius(0)
  .outerRadius(Math.min(width, height) / 2 - 20);

const colourScale = d3.scaleOrdinal(d3.schemeCategory10);

const g = svg.append("g")
  .attr("transform", `translate(${width / 2},${height / 2})`);

g.selectAll("path")
  .data(pie(data))
  .join("path")
  .attr("d", arc)
  .attr("fill", (d, i) => colourScale(i))
  .attr("stroke", "white")
  .attr("stroke-width", 2);
```

### Force-directed network

```javascript
const simulation = d3.forceSimulation(nodes)
  .force("link", d3.forceLink(links).id(d => d.id).distance(100))
  .force("charge", d3.forceManyBody().strength(-300))
  .force("center", d3.forceCenter(width / 2, height / 2));

const link = g.selectAll("line")
  .data(links)
  .join("line")
  .attr("stroke", "#999")
  .attr("stroke-width", 1);

const node = g.selectAll("circle")
  .data(nodes)
  .join("circle")
  .attr("r", 8)
  .attr("fill", "steelblue")
  .call(d3.drag()
    .on("start", dragstarted)
    .on("drag", dragged)
    .on("end", dragended));

simulation.on("tick", () => {
  link
    .attr("x1", d => d.source.x)
    .attr("y1", d => d.source.y)
    .attr("x2", d => d.target.x)
    .attr("y2", d => d.target.y);
  
  node
    .attr("cx", d => d.x)
    .attr("cy", d => d.y);
});

function dragstarted(event) {
  if (!event.active) simulation.alphaTarget(0.3).restart();
  event.subject.fx = event.subject.x;
  event.subject.fy = event.subject.y;
}

function dragged(event) {
  event.subject.fx = event.x;
  event.subject.fy = event.y;
}

function dragended(event) {
  if (!event.active) simulation.alphaTarget(0);
  event.subject.fx = null;
  event.subject.fy = null;
}
```

## Adding interactivity

### Tooltips

```javascript
// Create tooltip div (outside SVG)
const tooltip = d3.select("body").append("div")
  .attr("class", "tooltip")
  .style("position", "absolute")
  .style("visibility", "hidden")
  .style("background-color", "white")
  .style("border", "1px solid #ddd")
  .style("padding", "10px")
  .style("border-radius", "4px")
  .style("pointer-events", "none");

// Add to elements
circles
  .on("mouseover", function(event, d) {
    d3.select(this).attr("opacity", 1);
    tooltip
      .style("visibility", "visible")
      .html(`<strong>${d.label}</strong><br/>Value: ${d.value}`);
  })
  .on("mousemove", function(event) {
    tooltip
      .style("top", (event.pageY - 10) + "px")
      .style("left", (event.pageX + 10) + "px");
  })
  .on("mouseout", function() {
    d3.select(this).attr("opacity", 0.7);
    tooltip.style("visibility", "hidden");
  });
```

### Zoom and pan

```javascript
const zoom = d3.zoom()
  .scaleExtent([0.5, 10])
  .on("zoom", (event) => {
    g.attr("transform", event.transform);
  });

svg.call(zoom);
```

### Click interactions

```javascript
circles
  .on("click", function(event, d) {
    // Handle click (dispatch event, update app state, etc.)
    console.log("Clicked:", d);

    // Visual feedback
    d3.selectAll("circle").attr("fill", "steelblue");
    d3.select(this).attr("fill", "orange");

    // Optional: dispatch custom event for your framework/app to listen to
    // window.dispatchEvent(new CustomEvent('chartClick', { detail: d }));
  });
```

## Transitions and animations

Add smooth transitions to visual changes:

```javascript
// Basic transition
circles
  .transition()
  .duration(750)
  .attr("r", 10);

// Chained transitions
circles
  .transition()
  .duration(500)
  .attr("fill", "orange")
  .transition()
  .duration(500)
  .attr("r", 15);

// Staggered transitions
circles
  .transition()
  .delay((d, i) => i * 50)
  .duration(500)
  .attr("cy", d => yScale(d.value));

// Custom easing
circles
  .transition()
  .duration(1000)
  .ease(d3.easeBounceOut)
  .attr("r", 10);
```

## Scales reference

### Quantitative scales

```javascript
// Linear scale
const xScale = d3.scaleLinear()
  .domain([0, 100])
  .range([0, 500]);

// Log scale (for exponential data)
const logScale = d3.scaleLog()
  .domain([1, 1000])
  .range([0, 500]);

// Power scale
const powScale = d3.scalePow()
  .exponent(2)
  .domain([0, 100])
  .range([0, 500]);

// Time scale
const timeScale = d3.scaleTime()
  .domain([new Date(2020, 0, 1), new Date(2024, 0, 1)])
  .range([0, 500]);
```

### Ordinal scales

```javascript
// Band scale (for bar charts)
const bandScale = d3.scaleBand()
  .domain(['A', 'B', 'C', 'D'])
  .range([0, 400])
  .padding(0.1);

// Point scale (for line/scatter categories)
const pointScale = d3.scalePoint()
  .domain(['A', 'B', 'C', 'D'])
  .range([0, 400]);

// Ordinal scale (for colours)
const colourScale = d3.scaleOrdinal(d3.schemeCategory10);
```

### Sequential scales

```javascript
// Sequential colour scale
const colourScale = d3.scaleSequential(d3.interpolateBlues)
  .domain([0, 100]);

// Diverging colour scale
const divScale = d3.scaleDiverging(d3.interpolateRdBu)
  .domain([-10, 0, 10]);
```

## Best practices

### Data preparation

Always validate and prepare data before visualisation:

```javascript
// Filter invalid values
const cleanData = data.filter(d => d.value != null && !isNaN(d.value));

// Sort data if order matters
const sortedData = [...data].sort((a, b) => b.value - a.value);

// Parse dates
const parsedData = data.map(d => ({
  ...d,
  date: d3.timeParse("%Y-%m-%d")(d.date)
}));
```

### Performance optimisation

For large datasets (>1000 elements):

```javascript
// Use canvas instead of SVG for many elements
// Use quadtree for collision detection
// Simplify paths with d3.line().curve(d3.curveStep)
// Implement virtual scrolling for large lists
// Use requestAnimationFrame for custom animations
```

### Accessibility

Make visualisations accessible:

```javascript
// Add ARIA labels
svg.attr("role", "img")
   .attr("aria-label", "Bar chart showing quarterly revenue");

// Add title and description
svg.append("title").text("Quarterly Revenue 2024");
svg.append("desc").text("Bar chart showing revenue growth across four quarters");

// Ensure sufficient colour contrast
// Provide keyboard navigation for interactive elements
// Include data table alternative
```

### Styling

Use consistent, professional styling:

```javascript
// Define colour palettes upfront
const colours = {
  primary: '#4A90E2',
  secondary: '#7B68EE',
  background: '#F5F7FA',
  text: '#333333',
  gridLines: '#E0E0E0'
};

// Apply consistent typography
svg.selectAll("text")
  .style("font-family", "Inter, sans-serif")
  .style("font-size", "12px");

// Use subtle grid lines