var chart = new Chartist.Line('.ct-chart', {
  labels: ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday'],
  series: [
    [12, 9, 7, 8, 5],
    [2, 1, 3.5, 7, 3],
    [1, 3, 4, 5, 6]
  ]
});

var newdata = {
  labels: ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat'],
  series: [
    [34, 5, 2, 5, 4, 3],
    [2, 3, 4, 8, 1, 2],
    [5, 4, 3, 2, 1, 0.5]
  ]
};

chart.update(newdata);