d3.brushX() (brushY, or brush for rectangles) draws a draggable, resizable selection and reports event.selection in pixels, or null when cleared; invert the pixels through the scale, round them to snap, and filter. brush.move sets the selection from code:
<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="122" font-family="Arial" font-size="12"></svg>
<p id="out" style="font:14px Arial; margin:0"></p>
<script>
const { months } = prepareSales(SALES), parse = d3.utcParse('%Y-%m'), svg = d3.select('svg');
const x = d3.scaleUtc([parse('2025-01'), parse('2026-09')], [10, 610]);
const y = d3.scaleLinear([0, 400], [100, 4]);
const bars = svg.selectAll('rect').data(months).join('rect').attr('x', d => x(d.date) + 1)
.attr('width', d => x(d3.utcMonth.offset(d.date, 1)) - x(d.date) - 2)
.attr('y', d => y(d.units)).attr('height', d => 100 - y(d.units));
svg.append('g').attr('transform', 'translate(0,100)').call(d3.axisBottom(x));
const brush = d3.brushX().extent([[10, 0], [610, 100]]).on('brush end', ({ selection }) => {
const [from, to] = (selection ?? [0, 0]).map(px => d3.utcMonth.round(x.invert(px)));
const picked = months.filter(d => d.date >= from && d.date < to);
bars.attr('fill', d => picked.includes(d) ? '#e09a10' : '#b0bec5');
d3.select('#out').text(`${picked.length} months, ${d3.sum(picked, d => d.units)} units`);
});
svg.append('g').call(brush).call(brush.move, [x(parse('2025-11')), x(parse('2026-03'))]);
window.__done = true;
</script>
The selection snapped to November 2025 through February 2026. A brush on a small overview that sets a large chart's domain is the classic focus plus context pattern; Linked Charts links charts this way.
<!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; }
#out { font-size: 14px; }
</style>
<script src="https://cdn.jsdelivr.net/npm/d3@7.9.0/dist/d3.min.js"></script>
<svg viewBox="0 0 600 130" font-size="11"></svg>
<p id="out"></p>
<p>Drag across the bars to select months; drag the edges to resize, click outside to clear.</p>
<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 { months } = prepareSales(SALES), parse = d3.utcParse('%Y-%m'), svg = d3.select('svg');
const x = d3.scaleUtc([parse('2025-01'), parse('2026-09')], [10, 590]);
const y = d3.scaleLinear([0, 500], [104, 4]);
const bars = svg.selectAll('rect.bar').data(months).join('rect').attr('class', 'bar')
.attr('x', d => x(d.date) + 1).attr('width', d => x(d3.utcMonth.offset(d.date, 1)) - x(d.date) - 2)
.attr('y', d => y(d.units)).attr('height', d => 104 - y(d.units));
svg.append('g').attr('transform', 'translate(0,104)').call(d3.axisBottom(x));
const brushG = svg.append('g');
const brush = d3.brushX().extent([[10, 0], [590, 104]])
.on('brush end', ({ selection, sourceEvent, type }) => {
// Pixels → dates through the scale, rounded to whole months
const [from, to] = (selection ?? [0, 0]).map(px => d3.utcMonth.round(x.invert(px)));
const picked = months.filter(d => d.date >= from && d.date < to);
bars.attr('fill', d => picked.includes(d) ? '#e09a10' : '#b0bec5');
d3.select('#out').text(selection ? `${picked.length} months, ${d3.sum(picked, d => d.units)} units` : 'nothing selected');
// Snap the brush itself when the user lets go
if (type === 'end' && sourceEvent && selection) brushG.transition().call(brush.move, [from, to].map(x));
});
brushG.call(brush).call(brush.move, [x(parse('2025-11')), x(parse('2026-03'))]); // set from code
</script>