[Chartjs]-Using chartjs v2 to show categorical values on axis instead of numeric

If you want to change just the tick labels use the callback property of ticks. For example

...
options: {
  scales: {
    yAxes: [{
      ticks: {
        callback: function(value) {
          if (value === 20)
            return 'Low';
          else if (value === 50)
            return 'Medium';
          else if (value === 80)
            return 'High';
          else
            return '';
        }
      }
    }]
  }
}

Fiddle – http://jsfiddle.net/1gfu0tfg/