Hovering a 4-pixel dot is hard. d3.Delaunay.from(points, fx, fy) triangulates the points (with Delaunator 2,624 ) and find(x, y) walks to the nearest one, so every pixel belongs to a point's Voronoi cell, drawn faintly here. One dispatched pointermove shows the tooltip:
<script src="https://cdn.jsdelivr.net/npm/d3@7.9.0/dist/d3.min.js"></script>
<script src="booknest-sales.js"></script><script src="booknest-prep.js"></script>
<svg width="620" height="158" font-family="Arial" font-size="13"></svg>
<script>
const { sales } = prepareSales(SALES), svg = d3.select('svg');
const x = d3.scaleUtc(d3.extent(sales, d => d.date), [12, 600]);
const y = d3.scaleLinear([0, 110], [154, 4]), px = d => x(d.date), py = d => y(d.units);
const delaunay = d3.Delaunay.from(sales, px, py);
svg.append('path').attr('d', delaunay.voronoi([0, 0, 620, 158]).render())
.attr('fill', 'none').attr('stroke', '#dde3e6');
const dots = svg.selectAll('circle').data(sales).join('circle')
.attr('cx', px).attr('cy', py).attr('r', 4).attr('fill', d => d.book.color);
const tip = svg.append('text').attr('text-anchor', 'end');
svg.on('pointermove', event => {
const d = sales[delaunay.find(...d3.pointer(event))]; // nearest point
dots.attr('r', p => p === d ? 8 : 4);
tip.attr('x', px(d) - 12).attr('y', py(d) - 8).text(`${d.book.title}: ${d.units} units`);
});
const box = svg.node().getBoundingClientRect(); // simulate the pointer
svg.node().dispatchEvent(new PointerEvent('pointermove',
{ clientX: box.x + 450, clientY: box.y + 30 }));
window.__done = true;
</script>
The pointer lay in the cell of Small Steps to Big Summits' 100-unit launch month. find() takes a few steps, far cheaper than testing every circle, and also serves canvas charts (Canvas Rendering).
<!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; background: #fff; }
</style>
<script src="https://cdn.jsdelivr.net/npm/d3@7.9.0/dist/d3.min.js"></script>
<p><label><input type="checkbox" id="cells" checked> show Voronoi cells</label> · move the pointer anywhere</p>
<svg viewBox="0 0 600 180" font-size="13"></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 };
})();
// 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 { sales } = prepareSales(SALES), svg = d3.select('svg');
const x = d3.scaleUtc(d3.extent(sales, d => d.date), [12, 588]);
const y = d3.scaleLinear([0, 130], [174, 6]), px = d => x(d.date), py = d => y(d.units);
const delaunay = d3.Delaunay.from(sales, px, py); // triangulate once
const cells = svg.append('path').attr('d', delaunay.voronoi([0, 0, 600, 180]).render())
.attr('fill', 'none').attr('stroke', '#dde3e6');
const dots = svg.selectAll('circle').data(sales).join('circle')
.attr('cx', px).attr('cy', py).attr('r', 4).attr('fill', d => d.book.color);
const tip = svg.append('text').attr('font-weight', 'bold');
function focus([mx, my]) {
const d = sales[delaunay.find(mx, my)]; // walks to the nearest point
dots.attr('r', p => p === d ? 8 : 4);
const right = px(d) > 400;
tip.attr('x', px(d) + (right ? -12 : 12)).attr('y', py(d) - 8).attr('text-anchor', right ? 'end' : 'start')
.text(`${d.book.title}, ${d3.utcFormat('%b %Y')(d.date)}: ${d.units} units`);
}
svg.on('pointermove', event => focus(d3.pointer(event)));
d3.select('#cells').on('change', e => cells.attr('display', e.target.checked ? null : 'none'));
focus([x(new Date('2026-03-01')), y(98)]); // Summit's launch month
</script>