Fix issues from latest merge

src/main/java/de/tudresden/inf/mci/brailleplot/diagrams/LineChart.java

@@ -19,8 +19,7 @@ public class LineChart extends Diagram {
      * @param data The container, which holds the information about the datapoints.
      */
     public LineChart(final PointListContainer<PointList> data) {
-        Objects.requireNonNull(data);
-        mData = data;
+        super(data);
     }
 
     /**