Take a look at this http://www.liftarchitects.com/journal/2007/2/18/interactive-panels.html

Hi, I’m trying to have the circle radius respond to two points (more points later on) i.e. when a circle moves away from one point its radius shrinks until it is in range of the other point at which time it enlarges (inversed attribute controller equation). I have tried adding another distance attribute that responds to the new point but it seems as though I can only wire one attribute to the radius at a time. I want to be able to copy the circle many times so creating a new circle with an independet radius would be to time consuming. Any ideas?

[...] oftware : Rhino 3d and Grasshopper (Explicit History) Level : Intermediate Description : In this Rhino video tutorial we cover how to make scaling circular patterns based on the distance between points.  This is similar to a tutorial i built in 3ds Max with wire parameters. [...]