trendline

Trendline

Add trend lines to any chart. ~1KB gzipped, zero dependencies, works with your data format.

    │
  8 │          ●    .---●
    │        ●  .--'
  4 │       .-●'
    │    ●-'
  0 └───────────────────
    0    1    2    3    4

    ●  actual data
    -  trend line (calculated)

Try it on StackBlitz · npm · GitHub

Why Trendline?

• ~1KB minified + gzipped — smaller than most icons
• Zero dependencies — nothing to audit, no supply chain risk
• Works with your data — pass your objects directly, no array conversion
• TypeScript native — full type safety built-in, no @types needed
• Universal — ESM, CommonJS, browser, Node.js all supported
• R² included — know how well your trend line fits the data

Install

npm install trendline

Quick Start

import { createTrend } from 'trendline';

const data = [
  { month: 1, revenue: 120 },
  { month: 2, revenue: 150 },
  { month: 3, revenue: 170 },
  { month: 4, revenue: 200 },
];

// Pass your data with the key names you're already using
const trend = createTrend(data, 'month', 'revenue');

console.log(trend.slope);     // 26.5 (revenue increases ~26.5 per month)
console.log(trend.yStart);    // 96.5 (projected revenue at month 0)
console.log(trend.rSquared);  // 0.99 (excellent fit!)
console.log(trend.calcY(12)); // 414.5 (predicted revenue at month 12)

Use Cases

• Sales forecasting — project future revenue from historical data
• Stock/crypto trends — visualize price direction
• Fitness tracking — show weight loss/gain trends over time
• Performance metrics — CPU usage, response times, error rates
• Sensor data — temperature, humidity, pressure trends
• Any time-series data — if you can plot it, you can trend it

Works With

D3.js · Recharts · Chart.js · Victory · Nivo · ECharts · Plotly · Highcharts · Canvas · SVG · or any visualization library

Examples

Recharts (React)

import { createTrend } from 'trendline';
import { LineChart, Line, XAxis, YAxis } from 'recharts';

function SalesChart({ data }) {
  const trend = createTrend(data, 'day', 'sales');

  // Only need two points to draw a straight line
  const trendLine = [
    { day: data[0].day, sales: trend.calcY(data[0].day) },
    { day: data.at(-1).day, sales: trend.calcY(data.at(-1).day) },
  ];

  return (
    <LineChart width={600} height={300} data={data}>
      <XAxis dataKey="day" />
      <YAxis />
      <Line dataKey="sales" stroke="#8884d8" />
      <Line
        data={trendLine}
        dataKey="sales"
        stroke="#ff7300"
        strokeDasharray="5 5"
        dot={false}
      />
    </LineChart>
  );
}

D3.js

import { createTrend } from 'trendline';
import * as d3 from 'd3';

const data = [
  { date: 1, price: 100 },
  { date: 2, price: 120 },
  { date: 3, price: 115 },
  { date: 4, price: 140 },
  { date: 5, price: 150 },
];

const trend = createTrend(data, 'date', 'price');

// Your existing scales
const x = d3.scaleLinear().domain([1, 5]).range([0, width]);
const y = d3.scaleLinear().domain([80, 160]).range([height, 0]);

// Add trend line
svg.append('line')
  .attr('x1', x(1))
  .attr('y1', y(trend.calcY(1)))
  .attr('x2', x(5))
  .attr('y2', y(trend.calcY(5)))
  .attr('stroke', '#ff7300')
  .attr('stroke-dasharray', '5,5');

Chart.js

import { createTrend } from 'trendline';
import Chart from 'chart.js/auto';

const data = [
  { x: 1, y: 10 },
  { x: 2, y: 15 },
  { x: 3, y: 13 },
  { x: 4, y: 17 },
];

const trend = createTrend(data, 'x', 'y');

new Chart(ctx, {
  type: 'scatter',
  data: {
    datasets: [
      { label: 'Data', data },
      {
        label: `Trend (R²=${trend.rSquared.toFixed(2)})`,
        data: [
          { x: 1, y: trend.calcY(1) },
          { x: 4, y: trend.calcY(4) },
        ],
        type: 'line',
        borderDash: [5, 5],
      },
    ],
  },
});

Vanilla JavaScript (Canvas)

import { createTrend } from 'trendline';

const data = [
  { x: 0, y: 20 },
  { x: 50, y: 45 },
  { x: 100, y: 40 },
  { x: 150, y: 70 },
  { x: 200, y: 65 },
];

const trend = createTrend(data, 'x', 'y');
const ctx = canvas.getContext('2d');

// Draw data points
data.forEach(({ x, y }) => {
  ctx.beginPath();
  ctx.arc(x, 100 - y, 4, 0, Math.PI * 2);
  ctx.fill();
});

// Draw trend line
ctx.beginPath();
ctx.setLineDash([5, 5]);
ctx.moveTo(0, 100 - trend.calcY(0));
ctx.lineTo(200, 100 - trend.calcY(200));
ctx.stroke();

API

createTrend(data, xKey, yKey)

Parameter	Type	Description
data	Array<{ [key]: number }>	Your data array
xKey	string	Property name for X values
yKey	string	Property name for Y values

Returns:

Property	Type	Description
slope	number	Rate of change (rise over run)
yStart	number	Y-intercept (value when x = 0)
calcY(x)	function	Get Y value for any X
rSquared	number	Fit quality: 0 = poor, 1 = perfect

TypeScript

import { createTrend, TrendResult } from 'trendline';

const trend: TrendResult = createTrend(data, 'time', 'value');

Understanding R²

The rSquared value tells you how well the trend line fits your data:

R²	Fit Quality
0.9 - 1.0	Excellent — data follows a clear linear pattern
0.7 - 0.9	Good — trend line is meaningful
0.4 - 0.7	Moderate — some correlation
0.0 - 0.4	Weak — data may not be linear

Contributing

Found a bug or have a feature request? Open an issue or submit a PR.

License

MIT