S. Hong, N. Nikolov, and A. Tarassov, "A 2.5D Hierarchical Drawing of Directed Graphs", Journal of Graph Algorithms and Applications, Vol. 11, no. 2, pp. 371396, 2007.
We introduce a new graph drawing convention for 2.5D hierarchical
drawings of directed graphs. The vertex set is partitioned both into layers
of vertices drawn in parallel planes and into k >= 2 subsets, called walls,
and also drawn in parallel planes. The planes of the walls are perpen
dicular to the planes of the layers. We present a method for computing
such layouts and introduce five alternative algorithms for partitioning the
vertex set into walls which correspond to different aesthetic requirements.
We evaluate our method with an extensive computational study.

SeokHee Hong and Nikola S. Nikolov, Hierarchical Layout of Directed Graphs in Three Dimensions, Proceedings of Graph Drawing 2005, pp. 251261, Lecture Notes in Computer Science 3843, 2005.
We introduce a new graph drawing convention for 3D hierarchical drawings of directed graphs. The vertex set is partitioned into layers of vertices drawn in parallel planes. The vertex set is further partitioned into k >= 2 subsets, called walls. The layout consists of a set of parallel walls which are perpendicular to the set of parallel planes of the layers. We also outline a method for computing such layouts and introduce four alternative algorithms for partitioning the vertex set into walls which address different aesthetic requirements. 
SeokHee Hong and N. Nikolov, Layered Drawings of Directed Graphs in Three Dimensions, Proceedings of APVIS (Asia Pacific Symposium on Information Visualisation) 2005, pp. 6570, Vol. 45, CRPIT, 2005.
We introduce a new graph drawing convention for 3D layered drawings of directed graphs. The vertex set is partitioned into layers with all edges pointing in the same direction. The layers occupy parallel planes and vertices in each layer occupy two parallel lines. Thus, the traditional 2D layered drawing of a directed graph is split into two vertical walls each containing a 2D layered drawing. We outline a technique for 3D layered digraph drawing which is an extension to the wellknown Sugiyama method for 2D layered digraph drawing, and we also present some experimental results
