code: function(tick, index, ticks) {
  			var formattedTick = tick.format(this.displayFormat);
  			var tickOpts = this.options.ticks;
  			var callback = helpers.getValueOrDefault(tickOpts.callback, tickOpts.userCallback);
  
  			if (callback) {
  				return callback(formattedTick, index, ticks);
  			}
  			return formattedTick;
  		}
docs: Function to format an individual tick mark