Diagrams enable programmers to reason, debug, and communicate. However, constructing diagrams for programming language data is unnecessarily hard. We present a declarative DSL, Spytial, that captures the essential spatial features of data. We endow Spytial with a spatial semantics, mapping values to the 2D plane, and prove key properties. Spytial uses constraint-solving to make interactive renderings. We show how Spytial can be embedded in three very different languages: Python, Rust, and Pyret. We present a novel counterfactual debugging aid for diagramming errors, combining textual and visual output. We evaluate the language and system for expressiveness, performance, and diagnostic quality. Finally, we also show how Spytial can be used to construct values interactively and visually while preserving spatial constraints.

@article{paperbot3680,
  title = {Diagramming Program Values by Spatial Refinement},
  author = {Siddhartha Prasad and Michael Tu and Karan Kashyap and Tim Nelson and Shriram Krishnamurthi},
  journal = {Proceedings of the ACM on Programming Languages},
  volume = {10},
  number = {PLDI},
  year = {2026},
  doi = {10.1145/3808275}
}