Trajectories capture the movements of objects with multiple attributes. A visualization method called density maps shows trends in these trajectories. Density map creation involves aggregating smoothed trajectories in a density field and then visualizing the field. Users can explore attributes along trajectories by calculating a density field for multiple data subsets. The method then either combines these density fields into a new density field or visualizes them and then combines them. Using an interactive distribution map, users can define subsets and, supported by graphics hardware, get fast feedback for these computationally expensive density field calculations. Given the generic method and the lack of domain-specific assumptions, this method might also be applicable for trajectories in other domains.