PlantUML uses Graphviz/DOT to compute node positionning for every UML diagrams (except Sequence Diagrams and Activity Beta Diagrams). The fact that DOT computes automatically the position of node is a key feature, and algorithms implemented in DOT usually give very good result. However, there are some drawbacks of using DOT : the computation is done by an external program (dot.exe on Windows, dot on linux), and that means that :
PlantUML has to create a new process for each diagram, (and communication with this new process could be source of bugs).
Users have to install DOT on their system, in addition of Java.
PlantUML is not fully portable.
Smetana code has been integrated into PlantUML, so you can use the !pragma graphviz_dot jdot directive to force PlantUML to use the internal Java port of GraphViz/Dot, instead of the regular dot process.
!pragma graphviz_dot jdot
Foo1 --> Foo2
Foo1 --> Foo3
Foo1 ---> Foo4 : test 4
Foo1 ----> Foo5 : test 5
As you can see, some feature are not implemented yet (drawing of arrow heads, for example). Performance is not very good, but this is only an alpha version. The key point is that this image has been generated without launching Graphviz/DOT!