Large data repositories such as electronic journal databases, document corpora and wikis often organise their content into categories. Librarians, researchers, and interested users who wish to know the content distribution among different categories face the challenge of analysing large amounts of data. Information visualization can assist the user by shifting the analysis task to the human visual sub-system. In this paper we describe three visualization methods we have implemented, which help users understand category hierarchies and content distribution within large document repositories, and present an evaluation of these visualizations, pointing out each of their relative strengths for communicating information about the underlying category structure.