..via an accidental key stroke.. Note that it is not listed here on this keyboard help page:
Right now I have to exit LR. It looks like it's replacing or otherwise comendering the histogram. This looks like a cool addition although am not likely to use it myself (rarely need the accuracy but when I do, don't have the stones to trust especially an LCD monitor over physical test prints ). Anyhow the question is: What key do I keep accidentally pressing to get in this mode, and how do I get out of it once I'm in it?
These shortcut lists are more extensive: http://members.lightroomqueen.com/index.php?/Knowledgebase/List/Index/ 199/keyboard-shortcuts just in case you run into it again. I've put a PDF version up there for download too, in case it helps.
Europe, Middle East and Africa