A Declarative Grammar for Interactive Trajectory Visualization: Interaction As First-Class Component

Trajectory visualizations play a crucial role in urban computing, supporting tasks from analyzing road networks and vehicle movements to informing transport policy. However, creating such visualizations faces usability and flexibility limitations in trajectory visualization scenarios: limited usability due to insufficient trajectory-specific specialization, and limited flexibility due to tightly-coupled interactions that are not treated as first-class components. To address these challenges, we first conduct a systematic review of prior work and define a comprehensive design space for interactive trajectory visualization across three key dimensions: transformation, display, and interaction. We then present TrajGram, a declarative grammar built upon this design space that streamlines the creation of diverse interactive trajectory visualizations. TrajGram enables rapid prototyping with rich interactions grounded in real-world traffic analysis needs, while remaining extensible for complex scenarios. We demonstrate the effectiveness, usability, and learning curve of TrajGram through two usage scenarios and a user study.