code: function(npos, pos, radius){
        var diffx = npos.x - pos.x, 
            diffy = npos.y - pos.y, 
            diff = diffx * diffx + diffy * diffy;
        return diff <= radius * radius;
      }
docs: /*
  Method: contains
  
  Returns *true* if *pos* is contained in the area of the shape. Returns *false* otherwise.
  
  Parameters:
  
  npos - (object) An *x*, *y* object with the <Graph.Node> position.
  pos - (object) An *x*, *y* object with the position to check.
  radius - (number) The radius of the rendered circle.
  
  Example:
  (start code js)
  NodeHelper.circle.contains({ x: 10, y: 30 }, { x: 15, y: 35 }, 30); //true
  (end code)