Awarn - 4:44 pm on Nov 25, 2012 (gmt 0)
Your graph has a lot of similarities to mine. I have that same blue spike in the indexed pages at the same exact time. When your nonindexed pages climb in late April I instead had a spike in blocked in robots. However my robots.txt hadnt been changed since 2009 but had no prior issues. I have since stripped the robots.txt and it gradually came down but the not selected climbed proportionately. Just a question here, did you increase security on the server or anything? I have not figured out what triggered it.