This formal study proposes a transformational approach to the definition of general purpose visual languages based on hierarchical structures, addressing more specifically DTD visualization as its application area. We show that such visual languages can be constructed through progressive refinement of a syntax based on nested/juxtaposed rectangles. Several transformation stages, which can all be formally characterized, produce a high quality visual representation which expresses the fundamental properties of the original structure. Moreover, this approach opens some perspectives in proving visual properties through standard mathematical tools such as inductive proofs, thus establishing some practical links between visual language theory and classical language theory.