For me it's a simple undotree toggle:
;; undotree
(defun undo-tree-visualizer-toggle ()
(interactive)
(if (get-buffer undo-tree-visualizer-buffer-name)
(undo-tree-visualizer-quit)
(undo-tree-visualize)))
;; Represent undo-history as an actual tree (visualize with C-x u)
(setq undo-tree-mode-lighter "")
(require 'undo-tree)
(global-undo-tree-mode)
(global-set-key [f8] 'undo-tree-visualizer-toggle)