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.