My cross-platform desktop text editor, KeenWrite, allows users to define variables in an external YAML file. The editor calls out to Kroki[1] to convert text-based diagrams to SVG. The diagrams can reference variables and are rendered using EchoSVG[2].
KeenWrite[3] can produce PDF documentation from Markdown documents that has PlantUML diagrams with elements stored in an external, machine-readable file. Here are screenshots showing variables on the left, diagram text in the middle, and a real-time render on the right:
