i am new to the chart js and javascript. as i have faced the same problem, i wanted to display the sum of two values into the label, i got some solution for the same as below. maybe it can help you. check it out:

      tooltips: {
                mode: 'index',
                callbacks: {
                    // use the footer callback to display the sum of the items 
        showing in the tooltip
                    footer: function(tooltipitems, data) {
                        var sum = 0;

                        tooltipitems.foreach(function(tooltipitem) {
                            sum += data.datasets[tooltipitem.datasetindex].data[tooltipitem.index];
                        return 'sum: ' + sum;
                footerfontstyle: 'normal'
            hover: {
                mode: 'index',
                intersect: true

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