Plugins

Remarks#

Starting with v2.1.0, you can create plugins for chart.js! Plugin official documentation

Plugins Introduction

Plugins are a way for a developer to modify a chart as it is being created. Chart.js calls all plugins at the following chart states:

Start of initialization
End of initialization
Start of update
After the chart scales have calculated
Start of datasets update
End of datasets update
End of update (before render occurs)
Start of draw
End of draw
Before datasets draw
After datasets draw
Resize
Before an animation is started

Creating a plugin

To create a plugin, create a JavaScript object with appropriate named functions for any chart state you wish to modify (listed above). After you have your plugin object, pass it to Chart.pluginService.register(PLUGIN_OBJECT_NAME); to let Chart.js know to register the plugin.

Minimal Plugin Example

// Create the plugin object with functions for all the chart states
var simplePlugin = {
  beforeInit: function(chartInstance) {},
  afterInit: function(chartInstance) {},

  resize: function(chartInstance, newChartSize) {},

  beforeUpdate: function(chartInstance) {},
  afterScaleUpdate: function(chartInstance) {},
  beforeDatasetsUpdate: function(chartInstance) {},
  afterDatasetsUpdate: function(chartInstance) {},
  afterUpdate: function(chartInstance) {},

  // This is called at the start of a render. It is only called once, even if the animation will run for a number of frames. Use beforeDraw or afterDraw
  // to do something on each animation frame
  beforeRender: function(chartInstance) {},

  // Easing is for animation
  beforeDraw: function(chartInstance, easing) {},
  afterDraw: function(chartInstance, easing) {},

  // Before the datasets are drawn but after scales are drawn
  beforeDatasetsDraw: function(chartInstance, easing) {},
  afterDatasetsDraw: function(chartInstance, easing) {},

  destroy: function(chartInstance) {}
};

// Let Chart.js know about the new plugin
Chart.pluginService.register(simplePlugin);

Currently this minimal plugin does not do anything. To make this plugin useful one would need to add code to the functions that modifies the chart.

Draw Horizonal Lines

Create horizontal lines with a label. This could be used to show notable values in the chart data (e.g. min, max, average). JSFiddle Demo

var horizonalLinePlugin = {
  afterDraw: function(chartInstance) {
    var yScale = chartInstance.scales["y-axis-0"];
    var canvas = chartInstance.chart;
    var ctx = canvas.ctx;
    var index;
    var line;
    var style;

    if (chartInstance.options.horizontalLine) {
      for (index = 0; index < chartInstance.options.horizontalLine.length; index++) {
        line = chartInstance.options.horizontalLine[index];

        if (!line.style) {
          style = "rgba(169,169,169, .6)";
        } else {
          style = line.style;
        }

        if (line.y) {
          yValue = yScale.getPixelForValue(line.y);
        } else {
          yValue = 0;
        }

        ctx.lineWidth = 3;

        if (yValue) {
          ctx.beginPath();
          ctx.moveTo(0, yValue);
          ctx.lineTo(canvas.width, yValue);
          ctx.strokeStyle = style;
          ctx.stroke();
        }

        if (line.text) {
          ctx.fillStyle = style;
          ctx.fillText(line.text, 0, yValue + ctx.lineWidth);
        }
      }
      return;
    };
  }
};
Chart.pluginService.register(horizonalLinePlugin);

Credit to L Bahr for the example