/**
  * @return GoogleChartAxisCollection
  **/
 public function addAxis(GoogleChartAxis $axis)
 {
     $typeId = $axis->getType()->getId();
     if (isset($this->axes[$typeId])) {
         throw new WrongArgumentException('Axis already exists');
     }
     $this->axes[$typeId] = $axis;
     return $this;
 }