D3.js: Isolate Node Groups
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**What I'm trying to achieve:**

I would like to have groups of nodes, in a tree-like structure where each root is either the main root, or is a descendent of a leaf from another tree.

Generating what you see below is easy, but what I'd really like to see is complete circles around each root. However, since the nodes are repelling each other the below gaps are present between each cluster. *I assume the solution involves ignoring the repulsion caused by charges between leaves coming from different roots*.

![enter image description here][1]


  [1]: http://i.stack.imgur.com/RAxxo.png

**Is this possible?**

From reading the D3 docs, I found that unlike the dynamic linkDistance and linkStrength methods, node charge manipulations seem to be universal:

    "All nodes are assumed to be infinitesimal points with equal charge and mass."

If this is possible, can one of you geniuses please guide me in the right direction?