I am using PlantUML API to generate Activity Diagram.
I have used GraphViz before and used rank parameter to define layout
rank=LR [for left to right]
No, you cannot with current version (7876). You can use the following command with State, Class, Component or Usecase diagrams:
left to right direction
(see http://plantuml.sourceforge.net/usecase.html#Rankdir)
However, this feature will be added in next version.