public function toString()
 {
     $string = parent::toString();
     $string .= '&' . $this->axesCollection->toString();
     if ($this->style->hasStyles()) {
         $string .= '&' . $this->style->toString();
     }
     if ($this->labelStyle->hasStyles()) {
         $string .= '&' . $this->labelStyle->toString();
     }
     return $string;
 }