d3.area() fills between a baseline y0 and a top y1. d3.stack() computes each layer's [y0, y1] pairs from a table with a row per x value and a column per key; d3.index builds that table from BookNest's tidy rows, and value supplies 0 before Small Steps to Big Summits was released:
<script src="https://cdn.jsdelivr.net/npm/d3@7.9.0/dist/d3.min.js"></script>
<script src="booknest-sales.js"></script>
<svg width="620" height="176" font-family="Arial" font-size="12"></svg>
<script>
const parse = d3.utcParse('%Y-%m'), ids = SALES.books.map(b => b.id), svg = d3.select('svg');
const byMonth = d3.index(SALES.sales, d => d.month, d => d.book); // month -> book -> row
const series = d3.stack().keys(ids)
.value(([, books], id) => books.get(id)?.units ?? 0)(byMonth); // one layer per book
const x = d3.scaleUtc(d3.extent(byMonth.keys(), parse), [30, 610]);
const y = d3.scaleLinear([0, d3.max(series.at(-1), d => d[1])], [156, 5]).nice();
const area = d3.area().x(d => x(parse(d.data[0]))).y0(d => y(d[0])).y1(d => y(d[1]))
.curve(d3.curveMonotoneX);
svg.selectAll('path').data(series).join('path').attr('d', area)
.attr('fill', s => SALES.books[s.index].color);
svg.append('g').attr('transform', 'translate(0,156)').call(d3.axisBottom(x));
svg.append('g').attr('transform', 'translate(30,0)').call(d3.axisLeft(y).ticks(4));
</script>
Each layer carries key and index, each point its table row as data. Middle layers are hard to compare; d3.stackOffsetExpand shows shares instead, and d3.stackOffsetWiggle makes a streamgraph.
<!doctype html>
<style>
body { margin: 0; padding: 8px; background: #fafaf7; font: 12px system-ui, sans-serif; color: #263238; }
svg { width: 100%; max-width: 600px; display: block; }
</style>
<script src="https://cdn.jsdelivr.net/npm/d3@7.9.0/dist/d3.min.js"></script>
<p>Offset: <label><input type="radio" name="o" value="stackOffsetNone" checked> none</label>
<label><input type="radio" name="o" value="stackOffsetExpand"> expand (shares)</label>
<label><input type="radio" name="o" value="stackOffsetWiggle"> wiggle (streamgraph)</label></p>
<svg viewBox="0 0 600 190" font-size="11"></svg>
<script>
// BookNest sample data, invented for these examples (not real sales)
const SALES = (() => {
const books = [
{ id: 1, key: 'harbor', title: 'The Quiet Harbor', author: 'Elena Marsh', genre: 'Fiction', color: '#1f5f8b', price: 18.99, rating: 4.6 },
{ id: 2, key: 'deepweb', title: 'Patterns of the Deep Web', author: 'Ravi Menon', genre: 'Technology', color: '#5b3f99', price: 39.5, rating: 4.3 },
{ id: 3, key: 'saffron', title: 'Salt and Saffron', author: 'Leila Haddad', genre: 'Cooking', color: '#e09a10', price: 24, rating: 4.8 },
{ id: 4, key: 'summit', title: 'Small Steps to Big Summits', author: 'Tom Okafor', genre: 'Self-Help', color: '#3f7d3a', price: 14.99, rating: 4.1 },
{ id: 5, key: 'clock', title: "The Clockmaker's Paradox", author: 'Iris Vale', genre: 'Science Fiction', color: '#b5452f', price: 16.2, rating: 4.5 },
{ id: 6, key: 'glass', title: 'Gardens in Glass', author: 'June Park', genre: 'Gardening', color: '#2a9d8f', price: 22.5, rating: 4.4 },
];
const units = { // units sold per month, January 2025 to August 2026
1: [40, 47, 46, 47, 57, 51, 46, 54, 60, 60, 71, 111, 52, 42, 49, 56, 59, 48, 53, 51],
2: [40, 37, 35, 41, 48, 43, 40, 42, 50, 53, 53, 84, 42, 35, 38, 41, 45, 41, 46, 46],
3: [46, 45, 41, 46, 53, 49, 46, 45, 60, 51, 63, 100, 48, 48, 49, 48, 53, 49, 48, 52],
4: [100, 80, 72, 70, 66, 66], // released March 2026
5: [48, 47, 54, 52, 65, 55, 57, 58, 64, 66, 75, 124, 58, 49, 59, 56, 63, 57, 54, 57],
6: [31, 30, 28, 35, 31, 32, 29, 30, 41, 36, 45, 64, 31, 30, 30, 38, 40, 38, 36, 38],
};
const months = d3.utcMonths(new Date('2025-01-01'), new Date('2026-09-01')).map(d3.utcFormat('%Y-%m'));
const sales = books.flatMap(b => units[b.id].map((n, i) =>
({ month: months[i + months.length - units[b.id].length], book: b.id, units: n })));
const ratings = [[4, 8, 20, 87, 293], [6, 10, 22, 95, 147], [2, 3, 8, 45, 342], [2, 3, 8, 24, 28],
[4, 8, 20, 90, 228], [4, 7, 20, 82, 147]].map((counts, i) => ({ book: i + 1, counts })); // 1 to 5 stars
const regions = [{ id: '826', name: 'United Kingdom', units: 1480 }, { id: '276', name: 'Germany', units: 1120 },
{ id: '250', name: 'France', units: 760 }, { id: '372', name: 'Ireland', units: 540 },
{ id: '528', name: 'Netherlands', units: 410 }, { id: '724', name: 'Spain', units: 320 }];
const links = [[5, 1, 38], [1, 3, 24], [2, 5, 18], [3, 6, 21], [4, 1, 12], [2, 6, 9], [4, 3, 15], [5, 6, 14]]
.map(([source, target, value]) => ({ source, target, value })); // customers who bought both
return { books, sales, ratings, regions, links };
})();
const parse = d3.utcParse('%Y-%m'), ids = SALES.books.map(b => b.id), svg = d3.select('svg');
const byMonth = d3.index(SALES.sales, d => d.month, d => d.book); // month → book → row
const x = d3.scaleUtc(d3.extent(byMonth.keys(), parse), [36, 590]);
const gx = svg.append('g').attr('transform', 'translate(0,166)').call(d3.axisBottom(x));
const gy = svg.append('g').attr('transform', 'translate(36,0)');
function draw(offset) {
const series = d3.stack().keys(ids).offset(d3[offset])
.order(offset === 'stackOffsetWiggle' ? d3.stackOrderInsideOut : d3.stackOrderNone)
.value(([, books], id) => books.get(id)?.units ?? 0)(byMonth); // 0 before Summit's release
const y = d3.scaleLinear([d3.min(series, s => d3.min(s, d => d[0])), d3.max(series, s => d3.max(s, d => d[1]))], [166, 6]);
const area = d3.area().x(d => x(parse(d.data[0]))).y0(d => y(d[0])).y1(d => y(d[1]))
.curve(d3.curveMonotoneX);
svg.selectAll('path.layer').data(series, s => s.key).join('path').attr('class', 'layer')
.attr('fill', s => SALES.books[s.index].color)
.transition().duration(700).attr('d', area);
gy.call(d3.axisLeft(y).ticks(4, offset === 'stackOffsetExpand' ? '.0%' : 'd'))
.attr('opacity', offset === 'stackOffsetWiggle' ? 0 : 1);
}
d3.selectAll('input[name=o]').on('change', e => draw(e.target.value));
draw('stackOffsetNone');
</script>