画布背景

在某些用例中，您可能希望在整个画布上使用背景图像或颜色。没有内置支持，您可以通过编写自定义插件来实现此目的。

在下面两个示例插件中，您可以看到如何将颜色或图像绘制到画布上作为背景。这种为图表提供背景的方法只有在您想要导出具有特定背景的图表时才需要。对于正常使用，您可以使用 CSS (在新窗口打开) 更轻松地设置背景。

const config = { type: 'doughnut', data: data, options: { plugins: { customCanvasBackgroundColor: { color: 'lightGreen', } } }, plugins: [plugin], };

const config = {
  type: 'doughnut',
  data: data,
  options: {
    plugins: {
      customCanvasBackgroundColor: {
        color: 'lightGreen',
      }
    }
  },
  plugins: [plugin],
};

const data = { labels: [ 'Red', 'Blue', 'Yellow' ], datasets: [{ label: 'My First Dataset', data: [300, 50, 100], backgroundColor: [ 'rgb(255, 99, 132)', 'rgb(54, 162, 235)', 'rgb(255, 205, 86)' ], hoverOffset: 4 }] };

const data = {
  labels: [
    'Red',
    'Blue',
    'Yellow'
  ],
  datasets: [{
    label: 'My First Dataset',
    data: [300, 50, 100],
    backgroundColor: [
      'rgb(255, 99, 132)',
      'rgb(54, 162, 235)',
      'rgb(255, 205, 86)'
    ],
    hoverOffset: 4
  }]
};

// 注意：对插件代码的更改不会反映到图表上，因为插件是在图表构造时加载的，编辑器的更改只会触发 chart.update()。 const plugin = { id: 'customCanvasBackgroundColor', beforeDraw: (chart, args, options) => { const {ctx} = chart; ctx.save(); ctx.globalCompositeOperation = 'destination-over'; ctx.fillStyle = options.color || '#99ffff'; ctx.fillRect(0, 0, chart.width, chart.height); ctx.restore(); } };

// Note: changes to the plugin code is not reflected to the chart, because the plugin is loaded at chart construction time and editor changes only trigger an chart.update().
const plugin = {
  id: 'customCanvasBackgroundColor',
  beforeDraw: (chart, args, options) => {
    const {ctx} = chart;
    ctx.save();
    ctx.globalCompositeOperation = 'destination-over';
    ctx.fillStyle = options.color || '#99ffff';
    ctx.fillRect(0, 0, chart.width, chart.height);
    ctx.restore();
  }
};

const config = { type: 'doughnut', data: data, plugins: [plugin], };

const config = {
  type: 'doughnut',
  data: data,
  plugins: [plugin],
};

const data = { labels: [ 'Red', 'Blue', 'Yellow' ], datasets: [{ label: 'My First Dataset', data: [300, 50, 100], backgroundColor: [ 'rgb(255, 99, 132)', 'rgb(54, 162, 235)', 'rgb(255, 205, 86)' ], hoverOffset: 4 }] };

const data = {
  labels: [
    'Red',
    'Blue',
    'Yellow'
  ],
  datasets: [{
    label: 'My First Dataset',
    data: [300, 50, 100],
    backgroundColor: [
      'rgb(255, 99, 132)',
      'rgb(54, 162, 235)',
      'rgb(255, 205, 86)'
    ],
    hoverOffset: 4
  }]
};

// 注意：对插件代码的更改不会反映到图表上，因为插件是在图表构造时加载的，编辑器的更改只会触发 chart.update()。 const image = new Image(); image.src = 'https://chart.js.cn/img/chartjs-logo.svg'; const plugin = { id: 'customCanvasBackgroundImage', beforeDraw: (chart) => { if (image.complete) { const ctx = chart.ctx; const {top, left, width, height} = chart.chartArea; const x = left + width / 2 - image.width / 2; const y = top + height / 2 - image.height / 2; ctx.drawImage(image, x, y); } else { image.onload = () => chart.draw(); } } };

// Note: changes to the plugin code is not reflected to the chart, because the plugin is loaded at chart construction time and editor changes only trigger an chart.update().
const image = new Image();
image.src = 'https://chart.js.cn/img/chartjs-logo.svg';
const plugin = {
  id: 'customCanvasBackgroundImage',
  beforeDraw: (chart) => {
    if (image.complete) {
      const ctx = chart.ctx;
      const {top, left, width, height} = chart.chartArea;
      const x = left + width / 2 - image.width / 2;
      const y = top + height / 2 - image.height / 2;
      ctx.drawImage(image, x, y);
    } else {
      image.onload = () => chart.draw();
    }
  }
};