Center-of-mass interpretation for bipartite purity analysis of $N$-party   entanglement
  We provide a graphical description of the entanglement of pure-state multiparty systems based on an analogy between a bipartite purity analysis and the centroid of a collection of point masses. This description applies to quantum systems with $N$ parties, each with an arbitrary number of (discrete) states. The case of $N$ qubits is highlighted for simplicity. This geometric description illustrates some of the restrictions in the form of inequalities that apply to entanglement in multiparty systems. 
Miguel A. Alonso, Xiao-Feng Qian, and J.H. Eberly
quant-ph