Graph exactly like mine..I was hit on 30 sept.. my site is few years old and reputable biz, and it was just started to rank on first three positions for top keywords.
I stil rank no1 for main keyword and brand name,all pages are indexed, no manual message,
I thought that dupe content may be the issue because there was lots of errors in wmt title section - google was confused with some parameters.
I also noticed that that happend few days after I expanded site on other language with very similar content. But i understood that if site is localy connecetd with only one area there will be no problems.
I fixed that, placed few canonical tags and those errors are slowly dissapearing but still not totaly cleansed.
I have few signs of recovery however for those main keyords google is now showing tag pages but not main content page.