private double getPercentage(final TreeNode<ChartData> NODE) {
        List<TreeNode<ChartData>> siblings   = NODE.getSiblings();
        double         sum        = siblings.stream().map(node -> node.getItem()).mapToDouble(ChartData::getValue).sum();
        return Double.compare(sum, 0) == 0 ? 1.0 : NODE.getItem().getValue() / sum;
    }
Defines the root element of the tree
@param TREE
src/main/java/eu/hansolo/tilesfx/chart/SunburstChart.java