x-Axis ticks callback receives wrong value argument

[cnoelle]

Expected Behavior

We can specify a callback for the ticks of an axis in the form

ticks: {
    callback: function(value, index, values) {
        return value;
    }
}

The callback function should receive the default tick label as first argument and a numeric index as second (https://www.chartjs.org/docs/3.0.0-alpha/axes/labelling.html).

Current Behavior

For the x-Axis the callback function receives the index twice instead of value and index. For the y-Axis it works fine.

Steps to Reproduce

https://jsfiddle.net/fshu1xa8/

Environment

Chart.js version 3.0.0-beta.3 (works fine in 2.9.3, although the syntax needs to be adapted).

[etimberg]

This is due to

Chart.js/src/scales/scale.category.js

Lines 31 to 49 in a8a83d1

	buildTicks() {
	const me = this;
	const min = me.min;
	const max = me.max;
	const offset = me.options.offset;
	const ticks = [];
	let labels = me.getLabels();
	// If we are viewing some subset of labels, slice the original array
	labels = (min === 0 && max === labels.length - 1) ? labels : labels.slice(min, max + 1);
	me._valueRange = Math.max(labels.length - (offset ? 0 : 1), 1);
	me._startValue = me.min - (offset ? 0.5 : 0);
	for (let value = min; value <= max; value++) {
	ticks.push({value});
	}
	return ticks;
	}

. I believe it's intentional to have common code between the axes so we should document that this is how the category scale works.

[cnoelle]

Ok, thank you for the explanation, @etimberg . Makes sense for categorical labels, even though I find it a bit confusing to have a different behaviour than for linear scale, for instance.

On a related note, it also took me some time to understand that the default axis type for the x-axis is "catetegory" even if the labels are numeric. It could be helpful to have another example under the header "Scale options" for "Numeric X Axis" (there is already "Non Numeric Y Axis"): https://www.chartjs.org/samples/3.0.0-alpha/.