The interaction between a knowledge management systems and the users requires well-adapted visualization tools with graphical formalization of knowledge. The formalization is often theoretically based on graph-models. Yet, the best associated visual representations use trees but may be more limited than those with graphs. This paper gives an introduction to Atanor, a knowledge management system, whose graphical model for visualizing knowledge is tree-based. However this approach entails vertex redundancies. Consequently, we develop a new approach based on a layered digraph to solve this problem. Finally, we draw a comparison on an industrial example showing the advantages of the new model.