Chart.js/docs/axes/labelling.md

Labeling Axes

When creating a chart, you want to tell the viewer what data they are viewing. To do this, you need to label the axis.

Scale Title Configuration

The scale label configuration is nested under the scale configuration in the scaleLabel key. It defines options for the scale title. Note that this only applies to cartesian axes.

Name	Type	Default	Description
display	boolean	false	If true, display the axis title.
align	string	'center'	Alignment of the axis title. Possible options are 'start', 'center' and 'end'
labelString	string	''	The text for the title. (i.e. "# of People" or "Response Choices").
lineHeight	number|string	1.2	Height of an individual line of text (see MDN).
fontColor	Color	'#666'	Font color for scale title.
fontFamily	string	"'Helvetica Neue', 'Helvetica', 'Arial', sans-serif"	Font family for the scale title, follows CSS font-family options.
fontSize	number	12	Font size for scale title.
fontStyle	string	'normal'	Font style for the scale title, follows CSS font-style options (i.e. normal, italic, oblique, initial, inherit).
padding	number|object	4	Padding to apply around scale labels. Only top and bottom are implemented.

Creating Custom Tick Formats

It is also common to want to change the tick marks to include information about the data type. For example, adding a dollar sign ('$'). To do this, you need to override the ticks.callback method in the axis configuration. In the following example, every label of the Y axis would be displayed with a dollar sign at the front.

If the callback returns null or undefined the associated grid line will be hidden.

var chart = new Chart(ctx, {
    type: 'line',
    data: data,
    options: {
        scales: {
            y: {
                ticks: {
                    // Include a dollar sign in the ticks
                    callback: function(value, index, values) {
                        return '$' + value;
                    }
                }
            }
        }
    }
});

The third parameter passed to the callback function is an array of labels, but in the time scale, it is an array of {label: string, major: boolean} objects.