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

function init() {
  /**
   * This is the code to write the FishEye plugin :
   */
  
  (function(){

    // First, let's write a FishEye class.
    // There is no need to make this class global, since it is made to be used through
    // the SigmaPublic object, that's why a local scope is used for the declaration.
    // The parameter 'sig' represents a Sigma instance.
    var FishEye = function(sig) { 
      sigma.classes.Cascade.call(this);      // The Cascade class manages the chainable property
                                             // edit/get function.

      var self = this;                       // Used to avoid any scope confusion.
      var isActivated = false;               // Describes is the FishEye is activated.

      this.p = {                             // The object containing the properties accessible with
        radius: 200,                         // the Cascade.config() method.
        power: 2
      };

      function applyFishEye(mouseX, mouseY) {   // This method will apply a formula relatively to
                                                // the mouse position.
        var newDist, newSize, xDist, yDist, dist,
            radius   = self.p.radius,
            power    = self.p.power,
            powerExp = Math.exp(power);

        sig.graph.nodes.forEach(function(node) {
          xDist = node.displayX - mouseX;
          yDist = node.displayY - mouseY;
          dist  = Math.sqrt(xDist*xDist + yDist*yDist);

          if(dist < radius){
            newDist = powerExp/(powerExp-1)*radius*(1-Math.exp(-dist/radius*power));
            newSize = powerExp/(powerExp-1)*radius*(1-Math.exp(-dist/radius*power));

            if(!node.isFixed){
              node.displayX = mouseX + xDist*(newDist/dist*3/4 + 1/4);
              node.displayY = mouseY + yDist*(newDist/dist*3/4 + 1/4);
            }

            node.displaySize = Math.min(node.displaySize*newSize/dist,10*node.displaySize);
          }
        });
      };

      // The method that will be triggered when Sigma's 'graphscaled' is dispatched.
      function handler() {
        applyFishEye(
          sig.mousecaptor.mouseX,
          sig.mousecaptor.mouseY
        );
      }

      this.handler = handler;

      // A public method to set/get the isActivated parameter.
      this.activated = function(v) {
        if(v==undefined){
          return isActivated;
        }else{
          isActivated = v;
          return this;
        }
      };

      // this.refresh() is just a helper to draw the graph.
      this.refresh = function(){
        sig.draw(2,2,2);
      };
    };

    // Then, let's add some public method to sigma.js instances :
    sigma.publicPrototype.activateFishEye = function() {
      if(!this.fisheye) {
        var sigmaInstance = this;
        var fe = new FishEye(sigmaInstance._core);
        sigmaInstance.fisheye = fe;
      }

      if(!this.fisheye.activated()){
        this.fisheye.activated(true);
        this._core.bind('graphscaled', this.fisheye.handler);
        document.getElementById(
          'sigma_mouse_'+this.getID()
        ).addEventListener('mousemove',this.fisheye.refresh,true);
      }

      return this;
    };

    sigma.publicPrototype.desactivateFishEye = function() {
      if(this.fisheye && this.fisheye.activated()){
        this.fisheye.activated(false);
        this._core.unbind('graphscaled', this.fisheye.handler);
        document.getElementById(
          'sigma_mouse_'+this.getID()
        ).removeEventListener('mousemove',this.fisheye.refresh,true);
      }

      return this;
    };

    sigma.publicPrototype.fishEyeProperties = function(a1, a2) {
      var res = this.fisheye.config(a1, a2);
      return res == s ? this.fisheye : res;
    };
  })();

  /**
   * Now, let's use our plugin :
   */
  var sigInst = sigma.init(document.getElementById('sigma-example')).drawingProperties({
    defaultLabelColor: '#fff',
    defaultLabelSize: 14,
    defaultLabelBGColor: '#fff',
    defaultLabelHoverColor: '#000',
    labelThreshold: 6,
    defaultEdgeType: 'curve'
  }).graphProperties({
    minNodeSize: 0.5,
    maxNodeSize: 5,
    minEdgeSize: 1,
    maxEdgeSize: 1
  }).mouseProperties({
    maxRatio: 1,
    mouseEnabled: false
  });

  // (requires "sigma.parseGexf.js" to be executed)
  sigInst.parseGexf('/data/les_miserables.gexf');

  // Finally, let's activate the FishEye on our instance:
  sigInst.activateFishEye().draw();
}

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