node layout problem

[Ged Robinson <robinson.ged AT gmail.com>]

I have two types of node in my data.  "index" nodes, and other non-index nodes.

I want the index nodes to be fixed in place because they represent something which has a real location geographically.

The non-index nodes are associated at any time with only one index node, i.e., they change their connection, switching from index node to index node.  I want them to move to cluster around the index node they are currently attached to.

My problem is that when I use "layout.frozen" on the index nodes (they do all have "x" and "y" attributes) then the index nodes themselves appear correctly placed in relation to one another, but the non-index nodes are all repelled and form a large circle around the index nodes. like the image attached.

If I don't use "layout.frozen" with the index nodes then they have arbitrary positions on the graph (not what I want) but the non-index nodes do cluster nicely around the index node to which they are attached.

I can post some code if that would be useful, but it's really basic GraphStream code. 

Can anyone advise me?

Br,

Ged