Node.js 如何在节点中将图形渲染为图像

我想在服务器上以图像格式渲染堆叠条形图

其预期用途是推送到不支持SVG的twitter等服务。以及可部署到Heroku等服务的代码

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我已经试过了（他们的节点包非常过时，API文档也很差）。我也看过Google Graph、Chart.js和AnyChart，但它们不支持渲染我所看到的图像

Vega是一种可视化语法，一种用于创建、保存和共享交互式可视化设计的声明性格式。使用Vega，您可以用JSON格式描述数据可视化，并使用HTML5画布或SVG生成交互式视图

例如，使用可以使用以下代码将图表渲染为PNG文件：

// START vega-demo.js
var vega = require('vega')
var fs = require('fs')

var stackedBarChartSpec = require('./stacked-bar-chart.spec.json');

// create a new view instance for a given Vega JSON spec
var view = new vega
  .View(vega.parse(stackedBarChartSpec))
  .renderer('none')
  .initialize();

// generate static PNG file from chart
view
  .toCanvas()
  .then(function (canvas) {
    // process node-canvas instance for example, generate a PNG stream to write var
    // stream = canvas.createPNGStream();
    console.log('Writing PNG to file...')
    fs.writeFile('stackedBarChart.png', canvas.toBuffer())
  })
  .catch(function (err) {
    console.log("Error writing PNG to file:")
    console.error(err)
  });
// END vega-demo.js

// START stacked-bar-chart.spec.json 
{
  "$schema": "https://vega.github.io/schema/vega/v3.0.json",
  "width": 500,
  "height": 200,
  "padding": 5,

  "data": [
    {
      "name": "table",
      "values": [
        {"x": 0, "y": 28, "c":0}, {"x": 0, "y": 55, "c":1},
        {"x": 1, "y": 43, "c":0}, {"x": 1, "y": 91, "c":1},
        {"x": 2, "y": 81, "c":0}, {"x": 2, "y": 53, "c":1},
        {"x": 3, "y": 19, "c":0}, {"x": 3, "y": 87, "c":1},
        {"x": 4, "y": 52, "c":0}, {"x": 4, "y": 48, "c":1},
        {"x": 5, "y": 24, "c":0}, {"x": 5, "y": 49, "c":1},
        {"x": 6, "y": 87, "c":0}, {"x": 6, "y": 66, "c":1},
        {"x": 7, "y": 17, "c":0}, {"x": 7, "y": 27, "c":1},
        {"x": 8, "y": 68, "c":0}, {"x": 8, "y": 16, "c":1},
        {"x": 9, "y": 49, "c":0}, {"x": 9, "y": 15, "c":1}
      ],
      "transform": [
        {
          "type": "stack",
          "groupby": ["x"],
          "sort": {"field": "c"},
          "field": "y"
        }
      ]
    }
  ],

  "scales": [
    {
      "name": "x",
      "type": "band",
      "range": "width",
      "domain": {"data": "table", "field": "x"}
    },
    {
      "name": "y",
      "type": "linear",
      "range": "height",
      "nice": true, "zero": true,
      "domain": {"data": "table", "field": "y1"}
    },
    {
      "name": "color",
      "type": "ordinal",
      "range": "category",
      "domain": {"data": "table", "field": "c"}
    }
  ],

  "axes": [
    {"orient": "bottom", "scale": "x", "zindex": 1},
    {"orient": "left", "scale": "y", "zindex": 1}
  ],

  "marks": [
    {
      "type": "rect",
      "from": {"data": "table"},
      "encode": {
        "enter": {
          "x": {"scale": "x", "field": "x"},
          "width": {"scale": "x", "band": 1, "offset": -1},
          "y": {"scale": "y", "field": "y0"},
          "y2": {"scale": "y", "field": "y1"},
          "fill": {"scale": "color", "field": "c"}
        },
        "update": {
          "fillOpacity": {"value": 1}
        },
        "hover": {
          "fillOpacity": {"value": 0.5}
        }
      }
    }
  ]
}
// END stacked-bar-chart.spec.json

将输出PNG文件：

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我使用噩梦无头浏览器在Node.js下捕获图表、可视化和报告

使用“噩梦”可以使用Node.js下各种各样的基于浏览器的可视化框架，包括和，它们都很棒

实际上，我已经创建了一个名为“结束噩梦”的npm模块，它允许您通过向Node.js下提供一些数据和C3图表定义来呈现图表

按如下方式安装：

npm install --save c3-chart-maker

const c3ChartMaker = require('c3-chart-maker');

const yourData = ... your data ...
const chartDefinition = { ... c3 chart definition ... }
const outputFilePath = "your-chart-output-file.png";

c3ChartMaker(yourData, chartDefinition, outputFilePath)
    .then(() => { 
        console.log('Done');
    })
    .catch(err => {
        console.error(err);
    });

像这样使用它：

npm install --save c3-chart-maker

const c3ChartMaker = require('c3-chart-maker');

const yourData = ... your data ...
const chartDefinition = { ... c3 chart definition ... }
const outputFilePath = "your-chart-output-file.png";

c3ChartMaker(yourData, chartDefinition, outputFilePath)
    .then(() => { 
        console.log('Done');
    })
    .catch(err => {
        console.error(err);
    });

请查看图表示例并查看C3图表定义的外观

您还可以手动使用梦魇来捕获任何网页或基于浏览器的可视化

要安装噩梦，请执行以下操作：

npm install --save nightmare

下面是一个可以捕获网页的示例：

const Nightmare = require('nightmare');

// This is the web page to capture.
// It can also be a local web server! 
// Or serve from the file system using file://
const urlToCapture = "http://my-visualization.com"; 
const outputFilePath = "your-chart-output-file.png";

const nightmare = new Nightmare(); // Create Nightmare instance.
nightmare.goto(urlToCapture) // Point the browser at the requested web page.
        .wait("svg") // Wait until the specified HTML element appears on the screen. 
        .screenshot(outputImagePath) // Capture a screenshot to an image file.
        .end() // End the Nightmare session. Any queued operations are completed and the headless browser is terminated.
        .then(() => {
            console.log("Done!");
        })
        .catch(err => {
            console.error(err);
        });

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在我的书中，我还专门用了整整一章来阐述这一点。

Vega是我最喜欢的，它既有情节性的，也有我发现的几个不同的用于包装图形的库。起初，我和织女星搏斗，但通过使用Codepen上的操场，我能够找出哪里出了问题。现在它肯定是我的最爱：DAlso，在vega之上有一个更高级别的api。如果您首先想要简单性（而不是灵活性），那么值得检查一下