Problem is when doing adjustments the graph doesn't change. To evaluate the result of an adjustment we need to save the file and then close/reopen (or navigate to next picture / go back). This kills any possibility to do cancel the change via Ctrl-Z

When the screen image is updated the histogram needs to be also updated.
The best feature would be to see the graph updated dynamically while the Adjust dialog is open.
There was a similar suggestion opened for XnView: http://newsgroup.xnview.com/viewtopic.php?t=5541