Graph rewriting systems have long been around in theoretical computer science, and are used as a computing model in discrete mathematics, algebra, mathematical logic or even linguistics. Graph rewriting systems operate on graphs by substituting local patterns. Starting from an initial graph, the system iterates transformation rules in a non necessarily deterministic way. The combination of rule applications turn the study of such a system into an involved task requiring high level expertise. The inherent complexity of graph rewriting systems motivated the use of graphical diagrams, even by experts, to convey ideas and theoretical concepts. In these situations, users are not experts in theoretical computer science but rather focus on how a graph rewriting system behaves as a model of the problem they study. Questions emerging from the target application nevertheless translate into questions about the rewriting system. The goal of this work is to allow users to experiment with a graph rewriting system, and reason about the system or the model it supports based on graphical representations of all its components. Visually reasoning about the system, domain experts are able to abstract and refine models, use simulation and deduction to compute or check postulated properties and make testable predictions about possible outcomes. PORGY possesses genuine features and appears to be the first system to offer multiple coordinated views, allowing users to visualize and manipulate the system at both a global and local scale.