     Values to display as labels of the y-axis. If not specified, will display the
     lowest, the middle and the highest values.
     */
-    var yLabels: Array<Float>!
+    var yLabels: Array<Float>?
     
     /**
     Formatter for the labels on the y-axis.

    The values to display as labels on the x-axis. You can format these values with the `xLabelFormatter` attribute.
    As default, it will display the values of the series which has the most data.
    */
    var xLabels: Array<Float>!
    
    /**
    Formatter for the labels on the x-axis. The `index` represents the `xLabels` index, `value` its value:

            }
        }
        else {
            labels = yLabels!
        }
        
        let scaled = scaleValuesOnYAxis(labels)
