_tickFormatFunction(time, index, ticks, format) {
    const options = this.options;
    const formatter = options.ticks.callback;

    if (formatter) {
      return call(formatter, [time, index, ticks], this);
    }

    const formats = options.time.displayFormats;
    const unit = this._unit;
    const majorUnit = this._majorUnit;
    const minorFormat = unit && formats[unit];
    const majorFormat = majorUnit && formats[majorUnit];
    const tick = ticks[index];
    const major = majorUnit && majorFormat && tick && tick.major;

    return this._adapter.format(time, format || (major ? majorFormat : minorFormat));
  }
Function to format an individual tick mark
@param {number} time
@param {number} index
@param {object[]} ticks
@param {string|undefined} [format]
@return {string}
@private