A pen-based interactive editor for network diagrams is proposed. The network diagrams are composed of geometrical figures and connecting lines. The user may hand-sketch a first diagram and then, modify it by adding new components or by erasing, replacing or moving the existing ones. The machine beautifies the draft and updates the whole structure of the diagram when the user produces local modifications. Graphic communication implies that the layout of the diagram follows principles which are grounded on visual perception. The machine must be able to detect perceptual constraints (alignments, equality of sizes etc.) for beautification and updating. A Perceptually Structured Representation (PSR) of the diagram is built automatically. A formal model of diagram perception is defined as a set of rules which are applied during the global analysis following the local analysis where the figures are detected and recognized.