chartjs-to-image

Chart.js to Image

This is a wrapper for exporting Chart.js as an image. It works on the server side as well as client side (although on the client you may prefer to use toBase64Image).

The renderer is based on QuickChart, a free and open-source web service for generating static charts. View the main QuickChart repository here.

Installation

If you are using npm:

npm install chartjs-to-image

Usage

This library provides a ChartJsImage object. Import it, instantiate it, and set your Chart.js config:

const ChartJsImage = require('chartjs-to-image');

const myChart = new ChartJsImage();
myChart.setConfig({
  type: 'bar',
  data: { labels: ['Hello world', 'Foo bar'], datasets: [{ label: 'Foo', data: [1, 2] }] },
});

Write the image to disk:

myChart.toFile('/tmp/mychart.png');

Convert it to a data URL:

const dataUrl = await myChart.toDataUrl();

Or get a buffer that contains your chart image:

const buf = await myChart.toBinary();

ChartJsImage supports some additional functions using a third party rendering service. Use getUrl() on your ChartJsImage object to obtain a URL that will display your chart when visited:

console.log(myChart.getUrl());
// Prints:  https://quickchart.io/chart?c=%7Btype%3A%27bar%27%2Cdata%3A%7Blabels%3A%5B%27Hello+world%27%2C%27Foo+bar%27%5D%2Cdatasets%3A%5B%7Blabel%3A%27Foo%27%2Cdata%3A%5B1%2C2%5D%7D%5D%7D%7D&w=500&h=300&bkg=transparent&f=png

For larger charts, you may not want to encode the chart in the URL. Use getShortUrl() to get a fixed-length URL:

const url = await myChart.getShortUrl();
console.log(url);
// Prints: https://quickchart.io/chart/render/f-a1d3e804-dfea-442c-88b0-9801b9808401

All the above examples create this following Chart.js image:

Customizing your chart

setConfig(chart)

Use this config to customize the Chart.js config object that defines your chart. You must set this before creating any outputs!

setChartJsVersion(version: string)

Sets the version of Chart.js to use. Defaults to the latest version of Chart.js v2. Other valid version settings include: "3", "4", "3.9.1", "4.1.1", etc.

setWidth(width: int)

Sets the width of the chart in pixels. Defaults to 500.

setHeight(height: int)

Sets the height of the chart in pixels. Defaults to 300.

setFormat(format: string)

Sets the format of the chart. Defaults to png. svg and webp are also valid.

setBackgroundColor(color: string)

Sets the background color of the chart. Any valid HTML color works. Defaults to #ffffff (white). Also takes rgb, rgba, and hsl values.

setDevicePixelRatio(ratio: float)

Sets the device pixel ratio of the chart. This will multiply the number of pixels by the value. This is usually used for retina displays. Defaults to 1.0.

Getting outputs

There are two ways to get a URL for your chart object.

getUrl(): string

Returns a URL that will display the chart image when loaded.

getShortUrl(): Promise

Uses the quickchart.io web service to create a fixed-length chart URL that displays the chart image. The Promise resolves with a URL such as https://quickchart.io/chart/render/f-a1d3e804-dfea-442c-88b0-9801b9808401.

Note that short URLs expire after a few days for users of the free service. You can subscribe to keep them around longer.

toBinary(): Promise

Creates a binary buffer that contains your chart image.

toDataUrl(): Promise

Returns a base 64 data URL beginning with data:image/png;base64.

toFile(pathOrDescriptor: string): Promise

Creates a file containing your chart image.

More examples

Check out the examples/ directory to see other usage. Here's a simple test that uses some of the custom parameters:

const chart = new ChartJsImage();

chart.setConfig({
  type: 'bar',
  data: { labels: ['Hello world', 'Foo bar'], datasets: [{ label: 'Foo', data: [1, 2] }] },
});
chart.setWidth(500).setHeight(300).setBackgroundColor('transparent');

console.log(chart.getUrl());
// https://quickchart.io/chart?c=%7Btype%3A%27bar%27%2Cdata%3A%7Blabels%3A%5B%27Hello+world%27%2C%27Foo+bar%27%5D%2Cdatasets%3A%5B%7Blabel%3A%27Foo%27%2Cdata%3A%5B1%2C2%5D%7D%5D%7D%7D&w=500&h=300&bkg=transparent&f=png

chart.toFile('/tmp/test.png')

Here's a more complicated chart that includes some Javascript:

chart.setConfig({
  type: 'bar',
  data: {
    labels: ['January', 'February', 'March', 'April', 'May'],
    datasets: [
      {
        label: 'Dogs',
        data: [50, 60, 70, 180, 190],
      },
    ],
  },
  options: {
    scales: {
      yAxes: [
        {
          ticks: {
            callback: function (value) {
              return '$' + value;
            },
          },
        },
      ],
    },
  },
});
chart.setWidth(500).setHeight(300).setBackgroundColor('#0febc2');

const buf = await chart.toBinary();