code: function diamond(parent, bbox, node) {
    var w = (bbox.width * Math.SQRT2) / 2;
    var h = (bbox.height * Math.SQRT2) / 2;
    var points = [
      { x:  0, y: -h },
      { x: -w, y:  0 },
      { x:  0, y:  h },
      { x:  w, y:  0 }
    ];
    var shapeSvg = parent.insert("polygon", ":first-child")
      .attr("points", points.map(function(p) { return p.x + "," + p.y; }).join(" "));
  
    node.intersect = function(p) {
      return intersectPolygon(node, points, p);
    };
  
    return shapeSvg;
  }
docs: Circumscribe an ellipse for the bounding box with a diamond shape. I derived the function to calculate the diamond shape from: http://mathforum.org/kb/message.jspa?messageID=3750236