How to add custom label for line chart from chart.js?

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var ctx = document.getElementById('myChart').getContext('2d');

var myChart = new Chart(ctx, {
    type: 'line',
    data: {
        labels: ['January', 'February', 'March', 'April', 'May', 'June', 'July'],
        datasets: [{
            label: 'Custom Label',
            data: [10, 20, 15, 25, 30, 35, 40],
            borderColor: 'blue',
        }]
    },
    options: {
        tooltips: {
            callbacks: {
                title: function(tooltipItem, data) {
                    return 'Custom Label: ' + data.labels[tooltipItem[0].index];
                },
                label: function(tooltipItem, data) {
                    return 'Value: ' + data.datasets[tooltipItem.datasetIndex].data[tooltipItem.index];
                }
            }
        }
    }
});