Parallelism is one of the key performance sources in modern computer systems. When heuristics-based automatic parallelization fails to improve performance, a cumbersome and error-prone manual transformation is often required. As a solution, we propose an interactive visual approach building on the polyhedral model that visualizes exact dependencies and parallelism; decomposes and replays a complex automatically computed transformation step by step; and allows for directly manipulating the visual representation as a means of transforming the program with immediate feedback. User studies suggest that our visualization is understood by experts and nonexperts alike, and that it may favor an exploratory approach.