[Solved]-Chart.js - Formatting Y axis-Chart.js

Here you can find a good example of how to format Y-Axis value.

Also, you can use scaleLabel : "<%=value%>" that you mentioned, It basically means that everything between <%= and %> tags will be treated as javascript code (i.e you can use if statments...)

As Nevercom said the scaleLable should contain javascript so to manipulate the y value just apply the required formatting.

Note the the value is a string.

var options = {      
    scaleLabel : "<%= value + ' + two = ' + (Number(value) + 2)   %>"
};

jsFiddle example

if you wish to set a manual y scale you can use scaleOverride

var options = {      
    scaleLabel : "<%= value + ' + two = ' + (Number(value) + 2)   %>",

    scaleOverride: true,
    scaleSteps: 10,
    scaleStepWidth: 10,
    scaleStartValue: 0

};

jsFiddle example

For anyone using 3.X.X, here's the updated syntax to change y axis labels:

scales: {
  y: {
    ticks: {
      callback: (label) => `$ ${label}`,
    },
  },
},

Chart.js 2.X.X

I know this post is old. But if anyone is looking for more flexible solution, here it is

var options = {
        scales: {
            yAxes: [{
                ticks: {
                    beginAtZero: true,
                    callback: function(label, index, labels) {
                        return Intl.NumberFormat().format(label);
                        // 1,350

                        return Intl.NumberFormat('hi', { 
                            style: 'currency', currency: 'INR', minimumFractionDigits: 0, 
                        }).format(label).replace(/^(\D+)/, '$1 ');
                        // ₹ 1,350

                        // return Intl.NumberFormat('hi', {
                            style: 'currency', currency: 'INR', currencyDisplay: 'symbol', minimumFractionDigits: 2 
                        }).format(label).replace(/^(\D+)/, '$1 ');
                        // ₹ 1,350.00
                    }
                }
            }]
        }
    }

'hi' is Hindi. Check here for other locales argument
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl#Locale_identification_and_negotiation#locales_argument

for more currency symbol
https://www.currency-iso.org/en/home/tables/table-a1.html

scaleLabel : "<%= Number(value).toFixed(2).replace('.', ',') + ' $'%>"

An undocumented feature of the ChartJS library is that if you pass in a function instead of a string, it will use your function to render the y-axis's scaleLabel.

So while, "<%= Number(value).toFixed(2).replace('.',',') + ' $' %>" works, you could also do:

scaleLabel: function (valuePayload) {
    return Number(valuePayload.value).toFixed(2).replace('.',',') + '$';
}

If you're doing anything remotely complicated, I'd recommend doing this instead.