Requirements engineering visualization is a rapidly growing field of research; however, the specific characteristics of what makes for effective visualizations during a particular engineering phase have not yet been distinguished. Visualizations, when coupled with traditional practices, augment the ability of resulting requirements artifacts to reach a wide range of stakeholders and provide for a rapid and shared understanding of complex information. This paper represents a survey of the research papers presented during the REV workshops from 2006 to 2008 in order to ascertain how the research trends have evolved over the past few years. By examining approaches to requirements engineering visualization that have been proposed, in retrospect, we hope to show the areas of recent focus, as well as to discover those areas that may hold opportunities for further research with respect to the most commonly understood RE lifecycle phases and activities. In the process, we offer a preliminary classification scheme through which to categorize the various research efforts. Where none existed before, the resulted categorization enables a constructive discussion about the coverage of previous REV contributions from various perspectives, while discovering the gaps, and provides opportunities for further research with the understanding of the trends of applying visualization in requirements engineering research and practice.