Thanks.

 

This is the library: http://murderandcreate.com/physics/

 

And this is the example. Arboretum:

 

import traer.physics.*;

final float NODE_SIZE = 10;
final float EDGE_LENGTH = 20;
final float EDGE_STRENGTH = 0.2;
final float SPACER_STRENGTH = 1000;

ParticleSystem physics;
float scale = 1;
float centroidX = 0;
float centroidY = 0;

// PROCESSING /////////////////////////////////////

void setup()
{
  size( 400, 400 );
  smooth();
  strokeWeight( 2 );
  ellipseMode( CENTER );      
 
  physics = new ParticleSystem( 0, 0.1 );
 
  // Runge-Kutta, the default integrator is stable and snappy,
  // but slows down quickly as you add particles.
  // 500 particles = 7 fps on my machine
 
  // Try this to see how Euler is faster, but borderline unstable.
  // 500 particles = 24 fps on my machine
  //physics.setIntegrator( ParticleSystem.MODIFIED_EULER );
 
  // Now try this to see make it more damped, but stable.
  //physics.setDrag( 0.2 );
 
 
  textFont( loadFont( "Algerian-48.vlw" ),16 );
 
  initialize();
}

void draw()
{
  physics.tick();
  if ( physics.numberOfParticles() > 1 )
    updateCentroid();
  background( 255 );
  fill( 0 );
  text( "" + physics.numberOfParticles() + " PARTICLES\n" + (int)frameRate + " FPS", 10, 20 );
  translate( width/2 , height/2 );
  scale( scale );
  translate( -centroidX, -centroidY );
 
  drawNetwork(); 
}

void drawNetwork()
{     
  // draw vertices
  fill( 160 );
  noStroke();
  for ( int i = 0; i < physics.numberOfParticles(); ++i )
  {
    Particle v = physics.getParticle( i );
    ellipse( v.position().x(), v.position().y(), NODE_SIZE, NODE_SIZE );
  }

  // draw edges
  stroke( 0 );
  beginShape( LINES );
  for ( int i = 0; i < physics.numberOfSprings(); ++i )
  {
    Spring e = physics.getSpring( i );
    Particle a = e.getOneEnd();
    Particle b = e.getTheOtherEnd();
    vertex( a.position().x(), a.position().y() );
    vertex( b.position().x(), b.position().y() );
  }
  endShape();
}

void mousePressed()
{
  addNode();
}

void mouseDragged()
{
  addNode();
}

void keyPressed()
{
  if ( key == 'c' )
  {
    initialize();
    return;
  }
 
  if ( key == ' ' )
  {
    addNode();
    return;
  }
}

// ME ////////////////////////////////////////////

void updateCentroid()
{
  float
    xMax = Float.NEGATIVE_INFINITY,
    xMin = Float.POSITIVE_INFINITY,
    yMin = Float.POSITIVE_INFINITY,
    yMax = Float.NEGATIVE_INFINITY;

  for ( int i = 0; i < physics.numberOfParticles(); ++i )
  {
    Particle p = physics.getParticle( i );
    xMax = max( xMax, p.position().x() );
    xMin = min( xMin, p.position().x() );
    yMin = min( yMin, p.position().y() );
    yMax = max( yMax, p.position().y() );
  }
  float deltaX = xMax-xMin;
  float deltaY = yMax-yMin;
 
  centroidX = xMin + 0.5*deltaX;
  centroidY = yMin +0.5*deltaY;
 
  if ( deltaY > deltaX )
    scale = height/(deltaY+50);
  else
    scale = width/(deltaX+50);
}

void addSpacersToNode( Particle p, Particle r )
{
  for ( int i = 0; i < physics.numberOfParticles(); ++i )
  {
    Particle q = physics.getParticle( i );
    if ( p != q /*&& p != r*/ )
      physics.makeAttraction( p, q, -SPACER_STRENGTH, 20 );
  }
}

void makeEdgeBetween( Particle a, Particle b )
{
  physics.makeSpring( a, b, EDGE_STRENGTH, EDGE_STRENGTH, EDGE_LENGTH );
}

void initialize()
{
  physics.clear();
  physics.makeParticle();
}

void addNode()
{
  Particle p = physics.makeParticle();
  Particle q = physics.getParticle( (int)random( 0, physics.numberOfParticles()-1) );
  while ( q == p )
    q = physics.getParticle( (int)random( 0, physics.numberOfParticles()-1) );
  addSpacersToNode( p, q );
  makeEdgeBetween( p, q );
  p.position().set( q.position().x() + random( -1, 1 ), q.position().y() + random( -1, 1 ), 0 );
}