The related paper is published on W. Wang, G. Zhang and J. Lu, "Hierarchy Visualization for Group Recommender Systems," in IEEE Transactions on Systems, Man, and Cybernetics: Systems, vol. 49, no. 6, pp. 1152-1163, June 2019. doi: 10.1109/TSMC.2017.2699690 URL: https://ieeexplore.ieee.org/document/8053925