code: function(n) {
        return {
          x1: n.x - n.size,
          y1: n.y - n.size,
          x2: n.x + n.size,
          y2: n.y - n.size,
          height: n.size * 2
        };
      }
docs: Transforms a graph node with x, y and size into an
  axis-aligned square.
  
  @param  {object} A graph node with at least a point (x, y) and a size.
  @return {object} A square: two points (x1, y1), (x2, y2) and height.