PlantUML

PlantUML as tool to generate diagrams based on a textual description.

The tool is Java based, so you need a Java runtime environment to run PlantUML. Diagrams can be created in multiple output formats. PNG is the default format, but you can also SVG to have vector based diagrams which can be scaled and printed without any quality issue.

See MkDocs about how to integrate PlantUML diagrams in MkDocs.