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)