private void prepareData() {
        root     = getTreeNode().getTreeRoot();
        maxLevel = root.getMaxLevel();

        // Create map of all nodes per level
        levelMap.clear();
        for (int i = 0 ; i <= maxLevel ; i++) { levelMap.put(i, new ArrayList<>()); }
        root.stream().forEach(node -> levelMap.get(node.getDepth()).add(node));

        for (int level = 1 ; level < maxLevel ; level++) {
            List<TreeNode<ChartData>> treeNodeList = levelMap.get(level);
            treeNodeList.stream()
                        .filter(node -> node.getChildren().isEmpty())
                        .forEach(node ->node.addNode(new TreeNode<ChartData>(new ChartData("", 0, Color.TRANSPARENT), node)));
        }
    }
Defines the root element of the tree
@param TREE
src/main/java/eu/hansolo/tilesfx/chart/SunburstChart.java