With LayoutService.getInstance().layout(diagram, LayoutType.DEFAULT); it
works.
But if there are unvisible nodes in a diagram, there will be a nullpointer
exception in LayoutService.getLayoutNodes(DiagramEditPart,List). This is
why, such nodes don't have a GraphicalEditPart (I think :-)).
And an other little thing: In the Javadoc comment of layout() stands
"ILayoutType". But there is no such class or interface. It should be
"LayoutType", which exists.