With the increasing interest in metamodeling techniques for Domain Specific Modeling Languages (DSML) definition, there is a strong need to improve the language modeling process. One of the problems to solve is language evolution. Possible solutions include maximizing the reuse of metamodel patterns, composing them to form new, more expressive DSMLs. In this paper we improve the process of rapid prototyping of DSML graphical editors in meta-modeling tools, by defining composition rules for the graphical syntax layer. The goal is to provide formally defined operators to specify what happens to graphical mappings when their respective metamodels are composed. This improves reuse of Domain Specific Modeling Languages definitions and reduces development time.