d3.scaleTime() (local time) and d3.scaleUtc() are linear scales over Dates whose ticks fall on calendar boundaries, with a default format that prints the year at January and month names elsewhere. Parse and scale date-only data such as "2025-01" in UTC, so a reader in another time zone never sees a month slip back:
<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="148" font-family="Arial"></svg>
<script>
const parse = d3.utcParse('%Y-%m');
const months = d3.rollups(SALES.sales, v => d3.sum(v, d => d.units), d => d.month)
.map(([month, units]) => ({ date: parse(month), units })); // Section 6.5.3
const x = d3.scaleUtc([parse('2025-01'), parse('2026-09')], [34, 610]);
const y = d3.scaleLinear([0, d3.max(months, d => d.units)], [124, 6]).nice();
const svg = d3.select('svg');
svg.selectAll('rect').data(months).join('rect').attr('fill', '#1f5f8b')
.attr('x', d => x(d.date) + 1).attr('y', d => y(d.units))
.attr('width', d => x(d3.utcMonth.offset(d.date, 1)) - x(d.date) - 2)
.attr('height', d => 124 - y(d.units));
svg.append('g').attr('transform', 'translate(0,124)').call(d3.axisBottom(x));
svg.append('g').attr('transform', 'translate(34,0)').call(d3.axisLeft(y).ticks(4));
</script>
Each bar spans its own month (February's is narrower): d3.utcMonth.offset(date, 1) finds the next boundary, and intervals such as d3.utcMonth.every(3) can also be passed to axis.ticks(). The March 2026 jump is Small Steps to Big Summits launching, in the invented data.
<!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>
<svg viewBox="0 0 600 170"></svg>
<p>Hover a bar. Dates are parsed and scaled in UTC, so no reader sees a month slip back.</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 };
})();
const parse = d3.utcParse('%Y-%m');
const months = d3.rollups(SALES.sales, v => d3.sum(v, d => d.units), d => d.month)
.map(([month, units]) => ({ date: parse(month), units }));
const x = d3.scaleUtc([parse('2025-01'), parse('2026-09')], [36, 590]);
const y = d3.scaleLinear([0, d3.max(months, d => d.units)], [140, 10]).nice();
const svg = d3.select('svg');
svg.selectAll('rect').data(months).join('rect').attr('fill', '#1f5f8b')
.attr('x', d => x(d.date) + 1).attr('y', d => y(d.units))
.attr('width', d => x(d3.utcMonth.offset(d.date, 1)) - x(d.date) - 2) // to the next boundary
.attr('height', d => 140 - y(d.units))
.append('title').text(d => `${d3.utcFormat('%B %Y')(d.date)}: ${d.units} units`);
// Ticks fall on calendar boundaries: the year at January, month names elsewhere
svg.append('g').attr('transform', 'translate(0,140)').call(d3.axisBottom(x));
svg.append('g').attr('transform', 'translate(36,0)').call(d3.axisLeft(y).ticks(4));
svg.append('text').attr('x', x(parse('2026-03')) + 4).attr('y', y(months.find(m => +m.date === +parse('2026-03')).units) - 6)
.attr('fill', '#3f7d3a').text('Summit launches');
</script>