code: function diagonal(s, d) {
        return `M ${s.x} ${s.y}
                C ${(s.x)} ${(s.y + d.y)/2},
                  ${(s.x)} ${(s.y + d.y)/2},
                  ${d.x} ${d.y}`;
      }
docs: Creates a curved (diagonal) path from parent to the child nodes tree links