Hi Kasper,
PlantUML is an excellent tool for visualizing code structure, design,
behavior... For both designing it and documenting it.
I think its most interesting feature is that its API is text-based: You
literally "spell out" your diagram in text phrases, and its (Java-based)
engine will render it as a diagram (with the help of GraphViz).
And since it's text-based, there are extensions to text editors and document
processors (e.g., Word, LibreOffice Writer) that allow you to include the
PlantUML text in your document, then will render the diagrams in your
document for you automatically. No cut-and-paste!
A standardized linkage to PlantUML in Pharo would be an excellent tool -- I
hope you're able to pursue this and make it available on GitHub...