---
title: Advanced usage
anchor: advanced-usage
---
### Prototype methods
For each chart, there are a set of global prototype methods on the shared `ChartType` which you may find useful. These are available on all charts created with Chart.js, but for the examples, let's use a line chart we've made.
```javascript
// For example:
var myLineChart = new Chart(ctx, config);
```
#### .destroy()
Use this to destroy any chart instances that are created. This will clean up any references stored to the chart object within Chart.js, along with any associated event listeners attached by Chart.js.
This must be called before the canvas is reused for a new chart.
```javascript
// Destroys a specific chart instance
myLineChart.destroy();
```
#### .update(duration, lazy)
Triggers an update of the chart. This can be safely called after replacing the entire data object. This will update all scales, legends, and then re-render the chart.
```javascript
// duration is the time for the animation of the redraw in miliseconds
// lazy is a boolean. if true, the animation can be interupted by other animations
myLineChart.data.datasets[0].data[2] = 50; // Would update the first dataset's value of 'March' to be 50
myLineChart.update(); // Calling update now animates the position of March from 90 to 50.
```
#### .render(duration, lazy)
Triggers a redraw of all chart elements. Note, this does not update elements for new data. Use `.update()` in that case.
```javascript
// duration is the time for the animation of the redraw in miliseconds
// lazy is a boolean. if true, the animation can be interupted by other animations
myLineChart.render(duration, lazy);
```
#### .stop()
Use this to stop any current animation loop. This will pause the chart during any current animation frame. Call `.render()` to re-animate.
```javascript
// Stops the charts animation loop at its current frame
myLineChart.stop();
// => returns 'this' for chainability
```
#### .resize()
Use this to manually resize the canvas element. This is run each time the canvas container is resized, but you can call this method manually if you change the size of the canvas nodes container element.
```javascript
// Resizes & redraws to fill its container element
myLineChart.resize();
// => returns 'this' for chainability
```
#### .clear()
Will clear the chart canvas. Used extensively internally between animation frames, but you might find it useful.
```javascript
// Will clear the canvas that myLineChart is drawn on
myLineChart.clear();
// => returns 'this' for chainability
```
#### .toBase64Image()
This returns a base 64 encoded string of the chart in it's current state.
```javascript
myLineChart.toBase64Image();
// => returns png data url of the image on the canvas
```
#### .generateLegend()
Returns an HTML string of a legend for that chart. The legend is generated from the `legendCallback` in the options.
```javascript
myLineChart.generateLegend();
// => returns HTML string of a legend for this chart
```
#### .getElementAtEvent(e)
Calling `getElementAtEvent(event)` on your Chart instance passing an argument of an event, or jQuery event, will return the single element at the event position. If there are multiple items within range, only the first is returned
```javascript
myLineChart.getElementAtEvent(e);
// => returns the first element at the event point.
```
#### .getElementsAtEvent(e)
Looks for the element under the event point, then returns all elements at the same data index. This is used internally for 'label' mode highlighting.
Calling `getElementsAtEvent(event)` on your Chart instance passing an argument of an event, or jQuery event, will return the point elements that are at that the same position of that event.
```javascript
canvas.onclick = function(evt){
var activePoints = myLineChart.getElementsAtEvent(evt);
// => activePoints is an array of points on the canvas that are at the same position as the click event.
};
```
This functionality may be useful for implementing DOM based tooltips, or triggering custom behaviour in your application.
#### .getDatasetAtEvent(e)
Looks for the element under the event point, then returns all elements from that dataset. This is used internally for 'dataset' mode highlighting
```javascript
myLineChart.getDatasetAtEvent(e);
// => returns an array of elements
```
### External Tooltips
You can enable custom tooltips in the global or chart configuration like so:
```javascript
var myPieChart = new Chart(ctx, {
type: 'pie',
data: data,
options: {
tooltips: {
custom: function(tooltip) {
// tooltip will be false if tooltip is not visible or should be hidden
if (!tooltip) {
return;
}
// Otherwise, tooltip will be an object with all tooltip properties like:
// tooltip.caretHeight
// tooltip.caretPadding
// tooltip.chart
// tooltip.cornerRadius
// tooltip.fillColor
// tooltip.font...
// tooltip.text
// tooltip.x
// tooltip.y
// etc...
}
}
}
});
```
See `sample/line-customTooltips.html` for examples on how to get started.
### Writing new scale types
Starting with Chart.js 2.0 scales can be individually extended. Scales should always derive from Chart.Scale.
```javascript
var MyScale = Chart.Scale.extend({
/* extensions ... */
});
// MyScale is now derived from Chart.Scale
```
Once you have created your scale class, you need to register it with the global chart object so that it can be used. A default config for the scale may be provided when registering the constructor. The first parameter to the register function is a string key that is used later to identify which scale type to use for a chart.
```javascript
Chart.scaleService.registerScaleType('myScale', MyScale, defaultConfigObject);
```
To use the new scale, simply pass in the string key to the config when creating a chart.
```javascript
var lineChart = new Chart(ctx, {
data: data,
type: 'line',
options: {
scales: {
yAxes: [{
type: 'myScale' // this is the same key that was passed to the registerScaleType function
}]
}
}
})
```
#### Scale Properties
Scale instances are given the following properties during the fitting process.
```javascript
{
left: Number, // left edge of the scale bounding box
right: Number, // right edge of the bounding box'
top: Number,
bottom: Number,
width: Number, // the same as right - left
height: Number, // the same as bottom - top
// Margin on each side. Like css, this is outside the bounding box.
margins: {
left: Number,
right: Number,
top: Number,
bottom: Number,
},
// Amount of padding on the inside of the bounding box (like CSS)
paddingLeft: Number,
paddingRight: Number,
paddingTop: Number,
paddingBottom: Number,
}
```
#### Scale Interface
To work with Chart.js, custom scale types must implement the following interface.
```javascript
{
// Determines the data limits. Should set this.min and this.max to be the data max/min
determineDataLimits: function() {},
// Generate tick marks. this.chart is the chart instance. The data object can be accessed as this.chart.data
// buildTicks() should create a ticks array on the scale instance, if you intend to use any of the implementations from the base class
buildTicks: function() {},
// Get the value to show for the data at the given index of the the given dataset, ie this.chart.data.datasets[datasetIndex].data[index]
getLabelForIndex: function(index, datasetIndex) {},
// Get the pixel (x coordinate for horizontal scale, y coordinate for vertical scales) for a given value
// @param index: index into the ticks array
// @param includeOffset: if true, get the pixel halway between the given tick and the next
getPixelForTick: function(index, includeOffset) {},
// Get the pixel (x coordinate for horizontal scale, y coordinate for vertical scales) for a given value
// @param value : the value to get the pixel for
// @param index : index into the data array of the value
// @param datasetIndex : index of the dataset the value comes from
// @param includeOffset : if true, get the pixel halway between the given tick and the next
getPixelForValue: function(value, index, datasetIndex, includeOffset) {}
}
```
Optionally, the following methods may also be overwritten, but an implementation is already provided by the `Chart.Scale` base class.
```javascript
// Transform the ticks array of the scale instance into strings. The default implementation simply calls this.options.ticks.callback(numericalTick, index, ticks);
convertTicksToLabels: function() {},
// Determine how much the labels will rotate by. The default implementation will only rotate labels if the scale is horizontal.
calculateTickRotation: function() {},
// Fits the scale into the canvas.
// this.maxWidth and this.maxHeight will tell you the maximum dimensions the scale instance can be. Scales should endeavour to be as efficient as possible with canvas space.
// this.margins is the amount of space you have on either side of your scale that you may expand in to. This is used already for calculating the best label rotation
// You must set this.minSize to be the size of your scale. It must be an object containing 2 properties: width and height.
// You must set this.width to be the width and this.height to be the height of the scale
fit: function() {},
// Draws the scale onto the canvas. this.(left|right|top|bottom) will have been populated to tell you the area on the canvas to draw in
// @param chartArea : an object containing four properties: left, right, top, bottom. This is the rectangle that lines, bars, etc will be drawn in. It may be used, for example, to draw grid lines.
draw: function(chartArea) {},
```
The Core.Scale base class also has some utility functions that you may find useful.
```javascript
{
// Returns true if the scale instance is horizontal
isHorizontal: function(){},
// Get the correct value from the value from this.chart.data.datasets[x].data[]
// If dataValue is an object, returns .x or .y depending on the return of isHorizontal()
// If the value is undefined, returns NaN
// Otherwise returns the value.
// Note that in all cases, the returned value is not guaranteed to be a Number
getRightValue: function(dataValue){},
}
```
### Writing new chart types
Chart.js 2.0 introduces the concept of controllers for each dataset. Like scales, new controllers can be written as needed.
```javascript
Chart.controllers.MyType = Chart.DatasetController.extend({
});
// Now we can create a new instance of our chart, using the Chart.js API
new Chart(ctx, {
// this is the string the constructor was registered at, ie Chart.controllers.MyType
type: 'MyType',
data: data,
options: options
});
```
#### Dataset Controller Interface
Dataset controllers must implement the following interface.
```javascript
{
// Create elements for each piece of data in the dataset. Store elements in an array on the dataset as dataset.metaData
addElements: function() {},
// Create a single element for the data at the given index and reset its state
addElementAndReset: function(index) {},
// Draw the representation of the dataset
// @param ease : if specified, this number represents how far to transition elements. See the implementation of draw() in any of the provided controllers to see how this should be used
draw: function(ease) {},
// Remove hover styling from the given element
removeHoverStyle: function(element) {},
// Add hover styling to the given element
setHoverStyle: function(element) {},
// Update the elements in response to new data
// @param reset : if true, put the elements into a reset state so they can animate to their final values
update: function(reset) {},
}
```
The following methods may optionally be overridden by derived dataset controllers
```javascript
{
// Initializes the controller
initialize: function(chart, datasetIndex) {},
// Ensures that the dataset represented by this controller is linked to a scale. Overridden to helpers.noop in the polar area and doughnut controllers as these
// chart types using a single scale
linkScales: function() {},
// Called by the main chart controller when an update is triggered. The default implementation handles the number of data points changing and creating elements appropriately.
buildOrUpdateElements: function() {}
}
```
### Extending existing chart types
Extending or replacing an existing controller type is easy. Simply replace the constructor for one of the built in types with your own.
The built in controller types are:
* `Chart.controllers.line`
* `Chart.controllers.bar`
* `Chart.controllers.radar`
* `Chart.controllers.doughnut`
* `Chart.controllers.polarArea`
* `Chart.controllers.bubble`
#### Bar Controller
The bar controller has a special property that you should be aware of. To correctly calculate the width of a bar, the controller must determine the number of datasets that map to bars. To do this, the bar controller attaches a property `bar` to the dataset during initialization. If you are creating a replacement or updated bar controller, you should do the same. This will ensure that charts with regular bars and your new derived bars will work seamlessly.
### Building Chart.js
Chart.js uses gulp to build the library into a single JavaScript file.
Firstly, we need to ensure development dependencies are installed. With node and npm installed, after cloning the Chart.js repo to a local directory, and navigating to that directory in the command line, we can run the following:
```bash
npm install
npm install -g gulp
```
This will install the local development dependencies for Chart.js, along with a CLI for the JavaScript task runner gulp.
Now, we can run the `gulp build` task.
```bash
gulp build
```