/**
  * Returns minimum data value for a dataset
  */
 public function GetMinValue($dataset = 0)
 {
     if (!isset($this->min_value[$dataset])) {
         if (count($this->data[$dataset])) {
             $this->min_value[$dataset] = Graph::min($this->data[$dataset]);
         } else {
             $this->min_value[$dataset] = null;
         }
     }
     return $this->min_value[$dataset];
 }