public GroupLegendItem getLegendRoot() {
GroupLegendItem root = new GroupLegendItem();
GroupLegendItem group1 = new GroupLegendItem("Group 1");
group1.addGroupClass(NodeType1.class);
group1.addGroupClass(NodeType3.class);
root.add(group1);
GroupLegendItem group2 = new GroupLegendItem("Group 2");
group2.addGroupClass(NodeType2.class);
// group2.addGroupClass(GroupLegendItem.class);