Calculate value in tooltip in ChartJS

Question

I'm using chartJS to display data with 2 or sometimes 3 datasets.

Can I make it so it shows not only tooltipItem.yLabel, but also a percentage of total amount of yLabel (dataset1/(dataset1+dataset2))? I want to put this value in afterLabel.

ChartJS options code:

tooltips : {

    callbacks : {

        label : function(tooltipItem, data) {
            return data.datasets[tooltipItem.datasetIndex].label + ': ' + tooltipItem.yLabel;
        },
        afterLabel : function(tooltipItem, data) {
            return dataset1/(dataset1+dataset2);
        },
    }
}

My 'Y' datasets are arrays of numbers. X dataset is array of dates. I can't seem to figure out how chart.min.js takes these values.

Answer 1

I managed to do what I wanted, thank you @Kubilay Karpat for bringing me to an idea how to find needed values. I would +rep you, but I don't have enough to do so.

afterLabel : function(tooltipItem, data) {
    var total = 0;
    total = parseInt(data.datasets[0].data[tooltipItem.index]) + parseInt(data.datasets[1].data[tooltipItem.index]);                    
    var percentage = data.datasets[tooltipItem.datasetIndex].data[tooltipItem.index] / total * 100;
    var percentage = percentage.toFixed(2);
    return percentage + " %";
},