I am trying to create a force directed graph with toxiclibs, using VerletParticle2D and VerletSpring2D for my nodes and edges respectively.  My one problem is keeping the particles which aren't connected to each other by a spring from overlapping and hiding part of the data.  Is there a good way to avoid overlaps?  I don't need anything particularly "realistic," as far as collisions go, just a way to make sure all my particles are visible.

Toxiclibs is a wonderful library, and I am delighted to have found it.  Any help would be most appreciated.

FYI: At the moment the nodes are rendered as ellipses with their height and width based on the text displayed inside.

@toxi
Many thanks for the quick reply.  I had seen Shiffman's code and was adapting it for my project.  I tried the MinDistance spring as you suggested, but it didn't keep the nodes from overlapping -- maybe I'm doing something wrong.  I've attached the code (I switched back to the VerletSpring2D because with MinDistance the nodes settle too fast to see the physics behind the springs).  If you have a moment, could you run the program?  On most runs the graph settles with one of the nodes obscured.  I've commented out my partial attempt to surround each node with a CircularConstraint.  I wasn't sure if the constraint would move with the node.

Many, many thanks for your time.

import toxi.geom.*;
import toxi.physics2d.*;
import toxi.physics2d.constraints.*;

//reference to the physics world
VerletPhysics2D physics;

ArrayList particles;
ArrayList springs;

HashMap particleTable = new HashMap();

Particle rolloverItem;

PFont f;

void setup( ) {
 size(400, 400);
 smooth( );
 frameRate(30);
 f = createFont("GilSans",10, true);
 
 particles = new ArrayList();
 springs = new ArrayList();
 
 //Initialize the  physics
 physics = new VerletPhysics2D( );
 physics.setGravity(new Vec2D(0, 0.5));
 
 //This is the center of the world
 Vec2D center = new Vec2D(width/2, height/2);
 //these are the world's dimensions (33%, a vector point out from the center in both directions)
 Vec2D extent = new Vec2D(width/3, height/3);
 
 //Set the world's bounding box
 physics.setWorldBounds(Rect.fromCenterExtent(center, extent));
 
 loadData();

}

void loadData( ) {
 createParticle("John", "husband", "2");
 createParticle("Abigail", "wife", "1");
 createParticle("John Quincy", "husband", "3");
 createParticle("Louisa Catherine", "wife", "4");  
 createSpring("1","2");
 createSpring("1","3");
 createSpring("2","3");
 createSpring("3","4");
}


void draw( ) {
 //update the physics world
 physics.update( );
 background(255);
 stroke(0);
 
 rolloverItem = null;
 
 //draw a line between each particle that is connected by a spring
 for (int i = 0; i < springs.size(); i++) {
   VerletSpring2D s = (VerletSpring2D) springs.get(i);
   line(s.a.x, s.a.y, s.b.x, s.b.y);
 }
 
 //display all particles
 for (int i = 0; i < particles.size(); i++) {
   Particle p = (Particle) particles.get(i);
   p.display();
 }
 
/*
 if (rolloverItem !=null) {
   rolloverItem.drawName( );
 }
 if (mousePressed) {
   p1.drawName( );
 }
 */
}

Particle createParticle(String name, String type, String id) {
 Particle p = new Particle(Vec2D.randomVector(), 1, name, type, id);
 particles.add(p);
 particleTable.put(id, p);
 physics.addParticle(p);
 return p;
}

Particle findParticle(String id) {
   Particle p = (Particle) particleTable.get(id);
   if (p == null) {
     return createParticle("bad id", "dummy", id);
   }
   return p;
}

/*
CircularConstraint createConstraint(String id) {
 Particle p = findParticle(id);
 CircularConstraint c = new CircularConstraint(p.pos, 30);
 return c;
} */

VerletSpring2D createSpring(String id1, String id2) {
 Particle p1 = findParticle(id1);
 Particle p2 = findParticle(id2);
 VerletSpring2D s = new VerletSpring2D(p1, p2, 100, 0.01);
 springs.add(s);
 physics.addSpring(s);
 return s;
}


class Particle extends VerletParticle2D {
 
 String name;
 String type;
 String id;
 float w, h;
 
 Particle(Vec2D pos, float g, String name, String type, String id) {
   super(pos, g); //g for gravity
   this.name = name;
   this.type = type;
   this.id = id;
 }
 
 void display( ) {
   if (type == "husband") {
     fill(#009430); //green
   } else {
     fill(#e86a10); //orange
   }
   //fill(175);
   noStroke();
   
   textFont(f);
   float w = textWidth(name) + 15;
   float h = textAscent( ) + textDescent( ) + 10;
/*   if (w > h) {
     r = w;
   } else {
     r = h;
   } */
   ellipse(x, y, w, h);
   
   fill(0);
   textAlign(CENTER, CENTER);
   textFont(f);
   text(name, x, y);
   
   if (mouseInside()) {
     rolloverItem = this;
   }
 }
 
 void drawName( ) {
   fill(0);
   textAlign(CENTER, CENTER);
   textFont(f);
   text(name, x, y);
 }
 
 boolean mouseInside( ) {
   return (mouseX > x - w && mouseX < x + w &&
           mouseY > y - h && mouseY < y + h);
 }
 
 
   
}

Worked like a charm.  Thank you once again.