Chart.js Developer Guide (v4.5.1)

Advanced guide for creating custom plugins, extending Chart.js, and using the API.

Chart Instance API

Creating a Chart

const chart = new Chart(ctx, {
  type: 'bar',
  data: { /* ... */ },
  options: { /* ... */ }
});

Chart Methods

// Update chart
chart.update();                      // With animation
chart.update('none');                // Without animation
chart.update('active');              // Only animate active elements

// Data manipulation
chart.data.datasets[0].data.push(newValue);
chart.data.labels.push(newLabel);
chart.update();

// Resize
chart.resize();
chart.resize(width, height);

// Reset to original state
chart.reset();

// Destroy chart instance
chart.destroy();

// Convert to image
const base64Image = chart.toBase64Image();
const base64Image = chart.toBase64Image('image/png', 1.0);

// Show/hide datasets
chart.hide(datasetIndex);
chart.show(datasetIndex);
chart.isDatasetVisible(datasetIndex);
chart.setDatasetVisibility(datasetIndex, visible);

// Toggle data visibility (pie, doughnut, polar, bar)
chart.toggleDataVisibility(index);
chart.getDataVisibility(index);

// Animation control
chart.stop();                        // Stop current animation
chart.render();                      // Redraw without updating data
chart.clear();                       // Clear canvas

// Active elements
chart.setActiveElements([{ datasetIndex: 0, index: 1 }]);

// Dataset info
chart.getVisibleDatasetCount();
chart.getSortedVisibleDatasetMetas();

// Get elements at position
const elements = chart.getElementsAtEventForMode(
  event,
  'nearest',
  { intersect: true },
  true
);

// Get dataset metadata
const meta = chart.getDatasetMeta(datasetIndex);

Creating Custom Plugins

Plugins extend Chart.js functionality through lifecycle hooks.

Basic Plugin Structure

const myPlugin = {
  id: 'myPlugin',

  // Called when plugin is installed
  install(chart, args, options) {},

  // Called when plugin is started
  start(chart, args, options) {},

  // Called when plugin is stopped
  stop(chart, args, options) {},

  // Called when plugin is uninstalled
  uninstall(chart, args, options) {},

  // Lifecycle hooks
  beforeInit(chart, args, options) {},
  afterInit(chart, args, options) {},
  beforeUpdate(chart, args, options) {},
  afterUpdate(chart, args, options) {},
  beforeDraw(chart, args, options) {},
  afterDraw(chart, args, options) {},
  beforeDatasetsDraw(chart, args, options) {},
  afterDatasetsDraw(chart, args, options) {},
  beforeDatasetDraw(chart, args, options) {},
  afterDatasetDraw(chart, args, options) {},
  beforeEvent(chart, args, options) {},
  afterEvent(chart, args, options) {},
  beforeDestroy(chart, args, options) {},
  afterDestroy(chart, args, options) {},

  // Default options
  defaults: {
    color: 'red'
  }
};

Using Plugins

// Per-chart (inline plugin)
new Chart(ctx, {
  plugins: [myPlugin],
  options: {
    plugins: {
      myPlugin: {
        color: 'blue'
      }
    }
  }
});

// Global registration
Chart.register(myPlugin);

// Disable for specific chart
options: {
  plugins: {
    myPlugin: false
  }
}

Example: Canvas Background Plugin

const canvasBackgroundPlugin = {
  id: 'canvasBackground',

  beforeDraw(chart, args, options) {
    const { ctx, chartArea: { left, top, width, height } } = chart;

    ctx.save();
    ctx.fillStyle = options.color || 'white';
    ctx.fillRect(left, top, width, height);
    ctx.restore();
  },

  defaults: {
    color: 'white'
  }
};

// Usage
new Chart(ctx, {
  plugins: [canvasBackgroundPlugin],
  options: {
    plugins: {
      canvasBackground: {
        color: '#f0f0f0'
      }
    }
  }
});

Example: Chart Area Border Plugin

const chartAreaBorderPlugin = {
  id: 'chartAreaBorder',

  beforeDraw(chart, args, options) {
    const { ctx, chartArea: { left, top, width, height } } = chart;

    ctx.save();
    ctx.strokeStyle = options.borderColor || 'black';
    ctx.lineWidth = options.borderWidth || 1;
    ctx.setLineDash(options.borderDash || []);
    ctx.strokeRect(left, top, width, height);
    ctx.restore();
  },

  defaults: {
    borderColor: 'black',
    borderWidth: 1,
    borderDash: []
  }
};

Example: Crosshair Plugin

const crosshairPlugin = {
  id: 'crosshair',

  afterDraw(chart, args, options) {
    if (chart._active && chart._active.length) {
      const activePoint = chart._active[0];
      const { ctx } = chart;
      const { x, y } = activePoint.element;
      const { top, bottom, left, right } = chart.chartArea;

      ctx.save();
      ctx.beginPath();
      ctx.setLineDash([5, 5]);
      ctx.strokeStyle = options.color || 'gray';
      ctx.lineWidth = options.width || 1;

      // Vertical line
      ctx.moveTo(x, top);
      ctx.lineTo(x, bottom);

      // Horizontal line
      ctx.moveTo(left, y);
      ctx.lineTo(right, y);

      ctx.stroke();
      ctx.restore();
    }
  }
};

Plugin Lifecycle Hooks

Initialization Hooks

Hook	When Called
`beforeInit`	Before chart initializes
`afterInit`	After chart initializes

Update Hooks

Hook	When Called
`beforeUpdate`	Before chart updates
`afterUpdate`	After chart updates
`beforeLayout`	Before layout calculations
`afterLayout`	After layout calculations
`beforeDataLimits`	Before data limits are determined
`afterDataLimits`	After data limits are determined
`beforeBuildTicks`	Before ticks are built
`afterBuildTicks`	After ticks are built

Render Hooks

Hook	When Called
`beforeRender`	Before rendering starts
`afterRender`	After rendering completes
`beforeDraw`	Before chart draws
`afterDraw`	After chart draws
`beforeDatasetsDraw`	Before all datasets draw
`afterDatasetsDraw`	After all datasets draw
`beforeDatasetDraw`	Before each dataset draws
`afterDatasetDraw`	After each dataset draws

Event Hooks

Hook	When Called
`beforeEvent`	Before event is processed
`afterEvent`	After event is processed

Destruction Hooks

Hook	When Called
`beforeDestroy`	Before chart is destroyed
`afterDestroy`	After chart is destroyed

Accessing Chart Internals

// Chart instance
const chart = Chart.getChart('myChart');  // By canvas ID
const chart = Chart.getChart(canvasElement);  // By element

// Static methods
Chart.register(plugin);              // Register globally
Chart.unregister(plugin);            // Unregister globally

// Chart area bounds
const { left, top, right, bottom, width, height } = chart.chartArea;

// Canvas context
const ctx = chart.ctx;

// Scales
const xScale = chart.scales.x;
const yScale = chart.scales.y;

// Convert between pixel and data values
const xPixel = xScale.getPixelForValue(dataValue);
const xValue = xScale.getValueForPixel(pixelPosition);

// Dataset metadata
const meta = chart.getDatasetMeta(0);
const elements = meta.data;  // Array of visual elements

TypeScript Support

Basic Types

import { Chart, ChartConfiguration, ChartType } from 'chart.js';

const config: ChartConfiguration<'bar'> = {
  type: 'bar',
  data: {
    labels: ['A', 'B', 'C'],
    datasets: [{
      label: 'Dataset',
      data: [1, 2, 3]
    }]
  }
};

const chart = new Chart(canvas, config);

Custom Plugin Types

import { Plugin, ChartType } from 'chart.js';

interface MyPluginOptions {
  color: string;
  enabled: boolean;
}

const myPlugin: Plugin<ChartType, MyPluginOptions> = {
  id: 'myPlugin',
  beforeDraw(chart, args, options) {
    // options is typed as MyPluginOptions
  },
  defaults: {
    color: 'red',
    enabled: true
  }
};

// Extend Chart.js types
declare module 'chart.js' {
  interface PluginOptionsByType<TType extends ChartType> {
    myPlugin?: MyPluginOptions;
  }
}

Working with Canvas

const myPlugin = {
  id: 'customDraw',

  afterDraw(chart) {
    const { ctx, chartArea } = chart;

    ctx.save();

    // Draw custom shape
    ctx.fillStyle = 'rgba(255, 0, 0, 0.5)';
    ctx.beginPath();
    ctx.arc(
      chartArea.left + chartArea.width / 2,
      chartArea.top + chartArea.height / 2,
      50,
      0,
      Math.PI * 2
    );
    ctx.fill();

    // Draw text
    ctx.font = '16px Arial';
    ctx.fillStyle = 'black';
    ctx.textAlign = 'center';
    ctx.fillText('Custom Text', chart.width / 2, chart.height / 2);

    ctx.restore();
  }
};

Event Handling in Plugins

const eventPlugin = {
  id: 'eventHandler',

  beforeEvent(chart, args, options) {
    const event = args.event;

    if (event.type === 'click') {
      console.log('Click at:', event.x, event.y);
    }

    // Return false to stop event propagation
    // return false;
  },

  afterEvent(chart, args, options) {
    if (args.changed) {
      // Chart needs to re-render
    }
  }
};

Cancelling Plugin Execution

Return false from before* hooks to cancel:

const conditionalPlugin = {
  id: 'conditional',

  beforeDraw(chart, args, options) {
    if (someCondition) {
      return false;  // Cancel draw
    }
  }
};

Extending Chart.js

Custom Chart Types

Extend Chart.DatasetController to create new chart types. See references/custom-chart-types.md for:

DatasetController interface and required methods
Extending built-in controllers (BarController, LineController, etc.)
TypeScript typings for custom chart types

Custom Scales

Extend Chart.Scale to create custom axis types. See references/custom-scales.md for:

Scale interface and required methods
Scale properties available during fitting
Utility methods for pixel/value conversion

Updating Charts

Charts can be updated dynamically after creation. See references/updating-charts.md for:

Adding and removing data
Mutating vs replacing options
Update modes and animation control

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