The following code show how to write a plugin for sigma.js, and also shows how the FishEye works.

function init() {
  // The following method will parse the related sigma instance nodes
  // and set their positions around a circle:
  sigma.publicPrototype.myCircularLayout = function() {
    var R = 100,
        i = 0,
        L = this.getNodesCount();

    this.iterNodes(function(n){
      n.x = Math.cos(Math.PI*(i++)/L)*R;
      n.y = Math.sin(Math.PI*(i++)/L)*R;
    });

    return this.position(0,0,1).draw();
  };

  // The following method will parse the related sigma instance nodes
  // and set its position to as random in a square around the center:
  sigma.publicPrototype.myRandomLayout = function() {
    var W = 100,
        H = 100;
    
    this.iterNodes(function(n){
      n.x = W*Math.random();
      n.y = H*Math.random();
    });

    return this.position(0,0,1).draw();
  };

  // Instanciate sigma.js and customize it :
  var sigInst = sigma.init(document.getElementById('sigma-example')).drawingProperties({
    defaultLabelColor: '#fff'
  });

  // Generate a random graph with :
  //   . N nodes
  //   . E edges
  var i, N = 500, E = 3000;

  for(i = 0; i < N; i++){
    sigInst.addNode('n'+i,{
      'x': Math.random(),
      'y': Math.random(),
      'label': 'Node '+i,
      'size': 0.5+4.5*Math.random(),
      'color': 'rgb('+Math.round(Math.random()*256)+','+
                      Math.round(Math.random()*256)+','+
                      Math.round(Math.random()*256)+')'
    });
  }

  for(i = 0; i < E; i++){
    sigInst.addEdge(i,'n'+(Math.random()*N|0),'n'+(Math.random()*N|0));
  }

  // Draw the graph :
  sigInst.draw();

  // Finally, let's bind our methods to some buttons:
  document.getElementById('circular').addEventListener('click',function(){
    sigInst.myCircularLayout();
  },true);
  document.getElementById('random').addEventListener('click',function(){
    sigInst.myRandomLayout();
  },true);
}

if (document.addEventListener) {
  document.addEventListener('DOMContentLoaded', init, false);
} else {
  window.onload = init;
}