fix potential memory leak

[WolfgangFahl]

Exception in thread "JavaFX Application Thread" java.lang.OutOfMemoryError: Java heap space
	at java.util.HashMap.newNode(HashMap.java:1734)
	at java.util.HashMap.putVal(HashMap.java:630)
	at java.util.HashMap.put(HashMap.java:611)
	at java.util.HashSet.add(HashSet.java:219)
	at java.util.AbstractCollection.addAll(AbstractCollection.java:344)
	at java.util.HashSet.<init>(HashSet.java:119)
	at javafx.scene.chart.XYChart$Series$1.onChanged(XYChart.java:1457)