To cite package ‘plantuml’ in publications use:

  Krug R (2023). _plantuml: Create UML Graphs using PlantUML Syntax_. R
  package version 0.6.9, <https://github.com/rkrug/plantuml>.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {plantuml: Create UML Graphs using PlantUML Syntax},
    author = {Rainer M. Krug},
    year = {2023},
    note = {R package version 0.6.9},
    url = {https://github.com/rkrug/plantuml},
  }