d3.line(x, y) returns path data such as M0,0L10,5 for an array. One <path> draws the series, so bind it with datum(); curve() chooses how points connect, and defined() breaks the line at missing values:
<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="116" font-family="Arial" font-size="13"></svg>
<script>
const rows = SALES.sales.filter(s => s.book === 1)
.map(s => ({ date: d3.utcParse('%Y-%m')(s.month), units: s.units }));
const x = d3.scaleUtc(d3.extent(rows, d => d.date), [4, 146]);
const y = d3.scaleLinear(d3.extent(rows, d => d.units), [108, 26]);
const line = d3.line(d => x(d.date), d => y(d.units));
['curveLinear', 'curveMonotoneX', 'curveStep', 'curveBasis'].forEach((name, i) => {
const g = d3.select('svg').append('g').attr('transform', `translate(${i * 155}, 0)`);
g.append('text').attr('x', 4).attr('y', 14).text(name);
g.append('path').datum(rows).attr('d', line.curve(d3[name])) // d3.curveLinear, ...
.attr('fill', 'none').attr('stroke', '#1f5f8b').attr('stroke-width', 2);
g.selectAll('circle').data(rows).join('circle').attr('r', 2)
.attr('cx', d => x(d.date)).attr('cy', d => y(d.units));
});
</script>
curveMonotoneX smooths without overshooting, the safe choice for time series; curveStep suits values that hold until they change; the B-spline curveBasis misses the points and undershoots the December spike. curveCardinal and curveNatural can swing past the real extremes.
<!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 0 600 250" font-size="12"></svg>
<p><label><input type="checkbox" id="gap"> defined(): break the line at a missing month (June 2025)</label></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 rows = SALES.sales.filter(s => s.book === 1) // The Quiet Harbor
.map(s => ({ date: d3.utcParse('%Y-%m')(s.month), units: s.units }));
const x = d3.scaleUtc(d3.extent(rows, d => d.date), [8, 285]);
const y = d3.scaleLinear(d3.extent(rows, d => d.units), [110, 26]);
const line = d3.line(d => x(d.date), d => y(d.units)); // returns path data such as M8,99L22,86…
function draw() {
const gap = d3.select('#gap').property('checked');
line.defined(d => !(gap && d.date.getUTCMonth() === 5 && d.date.getUTCFullYear() === 2025));
d3.select('svg').selectAll('g.panel').data(['curveLinear', 'curveMonotoneX', 'curveStep', 'curveBasis'])
.join(enter => {
const g = enter.append('g').attr('class', 'panel');
g.append('text').attr('x', 8).attr('y', 14).attr('font-weight', 'bold').text(name => name);
g.append('path').attr('fill', 'none').attr('stroke', '#1f5f8b').attr('stroke-width', 2);
return g;
})
.attr('transform', (name, i) => `translate(${(i % 2) * 300}, ${Math.floor(i / 2) * 125})`)
.each(function (name) {
const g = d3.select(this);
g.select('path').datum(rows).attr('d', line.curve(d3[name])); // one path per series: datum()
g.selectAll('circle').data(rows).join('circle').attr('r', 2.5)
.attr('cx', d => x(d.date)).attr('cy', d => y(d.units)).attr('fill', '#263238');
});
}
d3.select('#gap').on('change', draw);
draw();
</script>