DSML (Domain Specific Modeling Languages) are an alternative to general purpose modeling languages (e.g. UML or SysML) for describing models with concepts and relations specific to a domain. DSML design is often based on Ecore metamodels, which follow the class-relation paradigm and also require defining a concrete syntax which can be either graphical or textual. In this paper, we focus on graphical concrete syntax, and we introduce an approach and a tool (Diagraph) to assist the design of a graphical DSML. The main principles are: non-intrusive annotations of the metamodel to identify nodes, edges, nesting structures and other graphical information; immediate validation of metamodels by immediate generation of an EMF-GMF instance editor supporting multi-diagramming. We report a comparison experience between Diagraph and Obeo Designer (a commercial proprietary tool), which was conducted as part of a Model Driven Engineering Course.