135 lines
5.3 KiB
Markdown
135 lines
5.3 KiB
Markdown
# New Axes
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Axes in Chart.js can be individually extended. Axes should always derive from `Chart.Scale` but this is not a mandatory requirement.
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```javascript
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let MyScale = Chart.Scale.extend({
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/* extensions ... */
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});
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// MyScale is now derived from Chart.Scale
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```
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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.
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```javascript
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Chart.scaleService.registerScaleType('myScale', MyScale, defaultConfigObject);
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```
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To use the new scale, simply pass in the string key to the config when creating a chart.
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```javascript
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var lineChart = new Chart(ctx, {
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data: data,
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type: 'line',
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options: {
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scales: {
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yAxes: [{
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type: 'myScale' // this is the same key that was passed to the registerScaleType function
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}]
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}
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}
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});
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```
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## Scale Properties
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Scale instances are given the following properties during the fitting process.
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```javascript
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{
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left: number, // left edge of the scale bounding box
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right: number, // right edge of the bounding box
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top: number,
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bottom: number,
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width: number, // the same as right - left
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height: number, // the same as bottom - top
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// Margin on each side. Like css, this is outside the bounding box.
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margins: {
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left: number,
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right: number,
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top: number,
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bottom: number
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},
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// Amount of padding on the inside of the bounding box (like CSS)
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paddingLeft: number,
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paddingRight: number,
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paddingTop: number,
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paddingBottom: number
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}
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```
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## Scale Interface
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To work with Chart.js, custom scale types must implement the following interface.
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```javascript
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{
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// Determines the data limits. Should set this.min and this.max to be the data max/min
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determineDataLimits: function() {},
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// Generate tick marks. this.chart is the chart instance. The data object can be accessed as this.chart.data
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// buildTicks() should create a ticks array on the axis instance, if you intend to use any of the implementations from the base class
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buildTicks: function() {},
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// 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]
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getLabelForIndex: function(index, datasetIndex) {},
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// Get the pixel (x coordinate for horizontal axis, y coordinate for vertical axis) for a given value
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// @param index: index into the ticks array
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getPixelForTick: function(index) {},
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// Get the pixel (x coordinate for horizontal axis, y coordinate for vertical axis) for a given value
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// @param value : the value to get the pixel for
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// @param index : index into the data array of the value
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// @param datasetIndex : index of the dataset the value comes from
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getPixelForValue: function(value, index, datasetIndex) {},
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// Get the value for a given pixel (x coordinate for horizontal axis, y coordinate for vertical axis)
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// @param pixel : pixel value
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getValueForPixel: function(pixel) {}
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}
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```
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Optionally, the following methods may also be overwritten, but an implementation is already provided by the `Chart.Scale` base class.
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```javascript
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{
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// Transform the ticks array of the scale instance into strings. The default implementation simply calls this.options.ticks.callback(numericalTick, index, ticks);
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convertTicksToLabels: function() {},
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// Determine how much the labels will rotate by. The default implementation will only rotate labels if the scale is horizontal.
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calculateTickRotation: function() {},
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// Fits the scale into the canvas.
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// 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.
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// 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
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// You must set this.minSize to be the size of your scale. It must be an object containing 2 properties: width and height.
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// You must set this.width to be the width and this.height to be the height of the scale
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fit: function() {},
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// 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
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// @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.
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draw: function(chartArea) {}
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}
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```
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The Core.Scale base class also has some utility functions that you may find useful.
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```javascript
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{
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// Returns true if the scale instance is horizontal
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isHorizontal: function() {},
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// Get the correct value from the value from this.chart.data.datasets[x].data[]
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// If dataValue is an object, returns .x or .y depending on the return of isHorizontal()
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// If the value is undefined, returns NaN
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// Otherwise returns the value.
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// Note that in all cases, the returned value is not guaranteed to be a number
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getRightValue: function(dataValue) {},
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// Returns the scale tick objects ({label, major})
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getTicks: function() {}
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}
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```
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