These last years have seen the multiplication of platforms dedicated to the conception and simulation of agent-based models for studying complex systems. If these platforms allowed to democratize this modeling approach, they are often complex to use by non-computer scientists as most of them require to define models by writing numerous code lines. In this paper, we present GAMAGraM, a new graphical modeling plug-in integrated into the GAMA platform. This plug-in allows users to define models from a graphical interface. Defining a model with the GAMAGraM plugin means first defining the structure of the model by a conceptual entity-relationship diagram, then defining the properties of entities through dedicated dialog boxes. The graphical editor provides autocompilation tools allowing to ease the work of the modelers.