refactor: add data set option for line tension

Controls the curve ratio for line charts. Line tension of 0
results in straight lines.

src/BaseDataset.php

@@ -134,6 +134,11 @@ abstract class BaseDataset extends \yii\base\Component
      */
     public $stack = null;
 
+    /**
+     * @var int
+     */
+    public $lineTension = 0;
+
     /**
      * Populates `$this->data` manipulating a query row into the correct format
      * for the dataset format use are using
@@ -219,6 +224,8 @@ public function getDataset()
             $dataset['stack'] = $this->stack;
         }
 
+        $dataset['lineTension'] = $this->lineTension;
+
         return $dataset;
     }