PlantUML uses an external layout engine for some diagrams (classes, use cases...). By default, GraphViz/dot is used. This means that PlantUML has to call an external process to generate some diagrams. There are some work in progress to change this behavior. For example, a port of GraphViz to Java is ongoing. Recently, Hallvard Trætteberg teaches us on how to use Eclipse Layout Kernel as a library to process the layout. This is still alpha-software, but results are very interesting. This is working starting from
This diagram is generated using ELK. Do not expect too much right now, this is really an alpha-version. The idea is to get feedback from users, to know if we should continue in this direction (that is, supporting ELK within PlantUML). Still, you can test class diagram and deployment diagram and tell us what you think about the result. Thanks for your attention !