callbacks: {
    label: function(tooltipItem, data) {
        var corporation = data.datasets[tooltipItem.datasetIndex].label;
        var valor = data.datasets[tooltipItem.datasetIndex].data[tooltipItem.index];

        // Loop through all datasets to get the actual total of the index
        var total = 0;
        for (var i = 0; i < data.datasets.length; i++)
            total += data.datasets[i].data[tooltipItem.index];

        // If it is not the last dataset, you display it as you usually do
        if (tooltipItem.datasetIndex != data.datasets.length - 1) {
            return corporation + " : $" + valor.toFixed(2).replace(/(\d)(?=(\d{3})+\.)/g, '$1,');
        } else { // .. else, you display the dataset and the total, using an array
            return [corporation + " : $" + valor.toFixed(2).replace(/(\d)(?=(\d{3})+\.)/g, '$1,'), "Total : $" + total];
        }
    }
}