/**
  * Factory method.
  *
  * @param int|null $xValue Optional x-value.
  * @param int|null $yValue Optional y-value.
  *
  * @return DataPointInterface New data point.
  */
 public static function factory($xValue = null, $yValue = null)
 {
     $dataPoint = new self();
     $dataPoint->setXValue($xValue)->setYValue($yValue);
     return $dataPoint;
 }