Chart Js Change Label orientation on x-Axis for Line Charts

If you are using chart.js 2.x, just set maxRotation: 90 and minRotation: 90 in ticks options. It works for me! And if you want to all x-labels, you may want to set autoSkip: false. The following is an example.

var myChart = new Chart(ctx, {
    type: 'bar',
    data: chartData,
    options: {
        scales: {
            xAxes: [{
                ticks: {
                    autoSkip: false,
                    maxRotation: 90,
                    minRotation: 90
                }
            }]
        }
    }
});