New Axes

Axes in Chart.js can be individually extended. Axes should always derive from Chart.Scale but this is not a mandatory requirement.

class MyScale extends Chart.Scale {
    /* extensions ... */
}
MyScale.id = 'myScale';
MyScale.defaults = defaultConfigObject;
// 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.

Chart.register(MyScale);
// If the new scale is not extending Chart.Scale, the prototype can not be used to detect what
// you are trying to register - so you need to be explicit:
// Chart.registry.addScales(MyScale);

To use the new scale, simply pass in the string key to the config when creating a chart.

const lineChart = new Chart(ctx, {
    data: data,
    type: 'line',
    options: {
        scales: {
            y: {
                type: 'myScale' // this is the same id that was set on the scale
            }
        }
    }
});

Scale Properties

Scale instances are given the following properties during the fitting process.

{
    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.

{
    // 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 axis instance, if you intend to use any of the implementations from the base class
    buildTicks: function() {},
    // Get the label to show for the given value
    getLabelForValue: function(value) {},
    // Get the pixel (x coordinate for horizontal axis, y coordinate for vertical axis) for a given value
    // @param index: index into the ticks array
    getPixelForTick: function(index) {},
    // Get the pixel (x coordinate for horizontal axis, y coordinate for vertical axis) for a given value
    // @param value : the value to get the pixel for
    // @param [index] : index into the data array of the value
    getPixelForValue: function(value, index) {},
    // Get the value for a given pixel (x coordinate for horizontal axis, y coordinate for vertical axis)
    // @param pixel : pixel value
    getValueForPixel: function(pixel) {}
}

Optionally, the following methods may also be overwritten, but an implementation is already provided by the Chart.Scale base class.

{
    // Adds labels to objects in the ticks array. The default implementation simply calls this.options.ticks.callback(numericalTick, index, ticks);
    generateTickLabels: function() {},
    // Determine how much the labels will rotate by. The default implementation will only rotate labels if the scale is horizontal.
    calculateLabelRotation: 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.

{
    // Returns true if the scale instance is horizontal
    isHorizontal: function() {},
    // Returns the scale tick objects ({label, major})
    getTicks: function() {}
}