画布背景

在某些用例中，你可能希望在整个画布上使用背景图片或颜色。 对此没有内置支持，你可以通过编写自定义插件来实现这一点。

在下面的两个示例插件中，你可以看到如何将颜色或图片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你想要导出具有该特定背景的图表时才有必要。 对于正常使用，你可以使用 CSS (opens new window) 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
  }]
};

// 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(); } };

// 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
  }]
};

// 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.nodejs.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.nodejs.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();
    }
  }
};