Thank you for the answer.
I use the answer-box rather than a comment because of the formating features.
My knowledge of DOT is from browsing the User's manual and a little bit of experimenting.
I'm probably wanting too much. I should accept this excerpt as an answer
2.6 Node and Edge Placement
[...]
Fine-tuning should be approached cautiously. dot works best when it can
makes a layout without much “help” or interference in its placement of individual
nodes and edges. Layouts can be adjusted somewhat by increasing the weight of
certain edges, or by creating invisible edges or nodes using style=invis, and
sometimes even by rearranging the order of nodes and edges in the file. But this can
backfire because the layouts are not necessarily stable with respect to changes in
the input graph. One last adjustment can invalidate all previous changes and make
a very bad drawing.
I had though that
subC -right-> Super2
subD -right-> Super2
subC -[hidden]down- subD
would produce three boxes arranged in a triangle as does the code below
subA -up-> Super1
subB -up-> Super1
subA -[hidden]right- subB
but rotated 90 degree.
After some trials and confusing results I produced the arrangement with dot:
@startuml
digraph foo {
rankdir=LR
subA;subB;Super;
subB->Super// [headport=nw;tailport=e];
subA->Super// [headport=sw;tailport=e];
subB->subA [headport=s;tailport=n;style=invis];
}
@enduml
I found that space matters!?! Changing "subB->Super" to "subB ->Super" produces a somewhat different result. The same applies to changing "subB->Super//" to "subB->Super //". The examples in the dot-user-guide have typically space before and after edgeop (arrows).
This simple example with only three nodes illustrates that it is close to impossible for the user to take control over the layout. It is dot that rules. Even if I found a robust way to produce the layout of three nodes I doubt that this way would work when the three nodes were included in a larger class diagram.
With less than ten classes, my reverse egineering tool together with PlantUML mostly produces useful class diagrams. And often a little interactive help (using [hidden],left,right,up,down) improves the readability. Thanks to PlantUML I've been able to make this useful tool. I recall "Perfect is the enemy of good" and will accept the DOT way of doing the layout.
Best regards
Per Isakson