Small multiples give each series a panel on shared scales; in Plot 40,840 that is two more channels: fx and fy split the data into columns and rows of facets, here a 3 x 2 grid:
<script src="https://cdn.jsdelivr.net/npm/d3@7.9.0/dist/d3.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/@observablehq/plot@0.6.17/dist/plot.umd.min.js">
</script>
<script src="booknest-sales.js"></script><script src="booknest-prep.js"></script>
<div id="chart"></div>
<script>
const { books, sales } = prepareSales(SALES);
const col = b => (b.id - 1) % 3, row = b => Math.floor((b.id - 1) / 3); // a 3 x 2 grid
const cell = { x: 'date', y: 'units', fx: d => col(d.book), fy: d => row(d.book) };
document.getElementById('chart').append(Plot.plot({
width: 620, height: 210, marginTop: 10, marginBottom: 34,
fx: { axis: null }, fy: { axis: null }, x: { ticks: 4, label: null },
y: { domain: [0, 120], ticks: 3, grid: true, label: null },
marks: [Plot.frame({ stroke: '#ccc' }),
Plot.areaY(sales, { ...cell, fill: d => d.book.color, fillOpacity: 0.25 }),
Plot.lineY(sales, { ...cell, stroke: d => d.book.color }),
Plot.text(books, { text: 'title', fx: col, fy: row, frameAnchor: 'top-left',
dx: 4, dy: 4 })
]
}));
</script>
Every mark repeats in every facet, filtered to the rows whose fx and fy match, so the text mark, fed the six books, labels the panels. A channel of CSS colors (d.book.color) is used as is, with no color scale.
<!doctype html>
<style>
body { margin: 0; padding: 8px; background: #fafaf7; font: 12px system-ui, sans-serif; color: #263238; }
</style>
<script src="https://cdn.jsdelivr.net/npm/d3@7.9.0/dist/d3.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/@observablehq/plot@0.6.17/dist/plot.umd.min.js"></script>
<p>Grid: <label><input type="radio" name="g" value="3" checked> 3 × 2</label> <label><input type="radio" name="g" value="2"> 2 × 3</label>
<label><input type="radio" name="g" value="6"> 6 × 1</label></p>
<div id="chart"></div>
<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 };
})();
// prepareSales(raw): parsed dates, sales linked to book objects, revenue and totals
function prepareSales(raw) {
const parse = d3.utcParse('%Y-%m'), books = raw.books.map(b => ({ ...b }));
const byId = d3.index(books, b => b.id);
const sales = raw.sales.map(s => ({ date: parse(s.month), book: byId.get(s.book),
units: s.units, revenue: s.units * byId.get(s.book).price }));
for (const [book, rows] of d3.group(sales, s => s.book)) {
book.units = d3.sum(rows, s => s.units);
book.revenue = d3.sum(rows, s => s.revenue);
}
const months = d3.rollups(sales, v => ({ units: d3.sum(v, s => s.units),
revenue: d3.sum(v, s => s.revenue) }), s => +s.date).map(([t, v]) => ({ date: new Date(t), ...v }))
.sort((a, b) => a.date - b.date);
const ratings = raw.ratings.map(r => ({ ...r, book: byId.get(r.book) }));
return { ...raw, books, sales, months, ratings };
}
const { books, sales } = prepareSales(SALES);
function draw(cols) {
const col = b => (b.id - 1) % cols, row = b => Math.floor((b.id - 1) / cols);
const cell = { x: 'date', y: 'units', fx: d => col(d.book), fy: d => row(d.book) }; // two more channels
const chart = Plot.plot({
width: 580, height: cols === 6 ? 130 : cols === 3 ? 230 : 330, marginTop: 10, marginBottom: 30,
fx: { axis: null }, fy: { axis: null }, x: { ticks: 3, label: null },
y: { domain: [0, 130], ticks: 3, grid: true, label: null },
marks: [
Plot.frame({ stroke: '#ccc' }),
Plot.areaY(sales, { ...cell, fill: d => d.book.color, fillOpacity: 0.25 }), // CSS colors used as is
Plot.lineY(sales, { ...cell, stroke: d => d.book.color }),
// The text mark gets the six books, so each facet gets its own label
Plot.text(books, { text: b => cols === 6 ? b.key : b.title, fx: col, fy: row, frameAnchor: 'top-left', dx: 4, dy: 4 }),
],
});
document.getElementById('chart').replaceChildren(chart);
}
document.querySelectorAll('input[name=g]').forEach(r => r.addEventListener('change', () => draw(+r.value)));
draw(3);
</script>