jharlap / echarts-node-export-server

Node.js server for offline export of ECharts graphs

Node.js server for offline export of ECharts graphs

This repository contains a Node.js application that starts a server which can render ECharts graphs to PNG images.

Requirements

This application needs two prerequisites:

• Node.js (preferably the latest LTS version)
• PhantomJS (installed via NPM)

Installation of Node.js

The installation of Node.js is covered in a separate document.

(If you prefer a setup within a Docker container over a native installation, then please take a look at Setup with Docker.)

Initial setup: install Node.js dependency packages

The application requires a package for PhantomJS. To install that, type

npm install

in the directory export-server/. You only need to do this once. The packages are then cached in the node_modules/ directory for future use.

However, PhantomJS does not offer a prebuilt package for some platforms, namely ARM which is used by Raspberry Pi (among others). In such case you can install the PhantomJS package provided by your package manager via

apt-get install phantomjs

on Raspbian, or via

yum install phantomjs

on RedHat-based distributions.

Start the application

You can simply start the application via

npm start

which fires up the Node.js application. The server will then listen on http://localhost:3000/ for incoming connections.

Usage

To generate a PNG file of an ECharts plot, just send an HTTP POST request to the running Node.js server on http://localhost:3000/ containing the data for the plot as JSON in its body.

For example, POSTing the following JSON code to the server

{
  "title": {
    "text": "ECharts entry example"
  },
  "tooltip": {},
  "legend": {
    "data": ["Sales"]
  },
  "xAxis": {
    "data": ["shirt","cardigan","chiffon shirt","pants","heels","socks"]
  },
  "yAxis": {},
  "series": [{
    "name": "Sales",
    "type": "bar",
    "data": [5,20,36,10,10,20]
  }]
}

will generate a JSON response like e.g.:

{
  "success": true,
  "filename": "graph-4a8d8e1d-eef8-4593-bd4c-422866243121.png"
}

The field filename in the response indicates the location of the generated image file on the server. In the example above you could then get the image from http://localhost:3000/graph-4a8d8e1d-eef8-4593-bd4c-422866243121.png. Given the data above the image would look like this:

Adjusting the width and height of the generated image

The generated PNG image has a width of 700 pixels and a height of 400 pixels by default. If no size is specified, then the image will be of that default size. However, this may not always be suitable for your needs.

There are two ways to change the image size:

• adding certain HTTP headers to the request (preferred way), or
• adding two data members to the POST-ed JSON data.

If both are present, then the HTTP headers take precedence.

Changing the size via HTTP headers

To change the size, add the HTTP headers X-Image-Width and / or X-Image-Height to the request. Acceptable values are integers only, the values will be interpreted as pixels, not centimetres, inches or other. For example, if you want the image to be 750 x 500 pixels, then set the HTTP headers

X-Image-Width: 750
X-Image-Height: 500

Using the same JSON as above, the created image will now be slightly larger and look like this:

Changing the size via JSON

If sending HTTP headers with your request is too cumbersome and you do not mind "polluting" the ECharts JSON with a bit of extra data, then that can be used to adjust the image size, too. Just add the members imageWidth and / or imageHeight to the top-level object. As with the HTTP headers, the values will be interpreted as pixels, not centimetres or inches.

For example, POSTing the following JSON code to the server

{
  "imageWidth": 750,
  "imageHeight": 500,
  "title": {
    "text": "ECharts entry example"
  },
  "tooltip": {},
  "legend": {
    "data": ["Sales"]
  },
  "xAxis": {
    "data": ["shirt","cardigan","chiffon shirt","pants","heels","socks"]
  },
  "yAxis": {},
  "series": [{
    "name": "Sales",
    "type": "bar",
    "data": [5,20,36,10,10,20]
  }]
}

will generate the same image with dimensions of 750 x 500 pixels.

Troubleshooting

If you encounter problems while trying to generate a chart image, then please take a look at the FAQ. Some common errors and possible solutions are listed there.

Version history

A version history is available in changelog.md.

Copyright and Licensing

Copyright 2018, 2020, 2021 Dirk Stolle

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.