code: function diagonal(s, d) {
      if (s.dy == undefined) {
          s.dy = 0;
      }
      if (d.dy == undefined) {
          d.dy = 0;
      }
      let path = `M ${s.x} ${(s.y + s.dy)}
            C ${(s.x + d.x) / 2} ${(s.y + s.dy)},
              ${(s.x + d.x) / 2} ${(d.y + d.dy)},
              ${d.x} ${(d.y + d.dy)}`;
      return path;
  }
docs: Create diagonal line between two nodes