Review diagram changes like code

Diagram-as-code tools have quietly become standard in engineering documentation. Mermaid, D2, Graphviz, and now Reladraw all let teams keep architecture diagrams in version control next to the code they describe. But there is a persistent complaint that surfaces in nearly every discussion thread about these tools: automatic layout is unpredictable. You change one line, and the entire graph rearranges itself. A node that sat comfortably in the top-left corner jumps to the bottom-right, edges cross in new places, and suddenly a two-line change produces a diff that looks like the whole diagram was rewritten.

Existing solutions do not address this. Mermaid's live editor shows you the current render but has no concept of a previous version. D2 ships a text-level diff command that tells you which lines changed, not which nodes moved. Graphviz can diff source files, but the output is a wall of text that tells you nothing about visual layout. None of these tools answer the question a reviewer actually asks: what changed in this picture, and does it matter? That gap is exactly where DiagramDiff sits.

The timing is right because the underlying signal is fresh. Reladraw recently appeared on Hacker News with a clear thesis — you decide where things go, not the layout engine — and it drew substantial discussion from developers who have been frustrated by automatic layout for years. That conversation confirms the pain is real and widely shared. DiagramDiff does not compete with these languages; it sits downstream of them, turning their output into something a team can actually review. The window is clean, the pain is documented, and the workflow gap is obvious to anyone who has ever tried to approve a diagram change in a pull request.