generateTickLabels(ticks) {
    const tickOpts = this.options.ticks;
    let i, ilen, tick;
    for (i = 0, ilen = ticks.length; i < ilen; i++) {
      tick = ticks[i];
      tick.label = call(tickOpts.callback, [tick.value, i, ticks], this);
    }
  }
Convert ticks to label strings
@param {Tick[]} ticks