@Whitey, I got hit on April 24 - that is beyond dispute. @Fathom believes that I was not hit by Penguin. I had no manual penalty applied, so whatever hit me was algorithmic. If you go with @Fathom's theory - that it wasn't Penguin - that means that if he's right, other sites could be chasing phantoms too - that they were hit by Panda coincidentally on the Penguin day. I don't know how anyone can validate that theory though.
I do get a lot of branded queries, and my site name comes up in the Google autocomplete quite a bit. I also have a weird phenomenon in that although 65% of my traffic is organic, 60% to 70% of my traffic is from repeat visitors. I don't know what to make of that.
The reason I float my theory is that the 2nd best league in my niche sport happened to start on October 12. It was an off-site event which I'd expect to naturally increase traffic. However the jump was more than expected, and was clearly the result of the removal of the penalty - I verified that with certain queries that were being capped at #11 but went to #1 after Oct 13. My graphs show the same thing - a new plateau was hit on Oct 13. April 24 did not correspond to an off-site event though, that is why the April drop makes no sense.
A possible explanation is that there is a threshold that beats penalties. Google claims to give the people what they want. If people are searching for "widgetmania" and Google is suppressing them at #11 because of an algorithmic penalty, maybe if they see enough people asking for "widgetmania" they give that site a get out of jail free card. We'll see what happens this summer, I guess.
@Fathom is correct in saying that a lot of my pages might appear thin to Google. Those pages definitely benefit from being associated with my brand, but it's long-tail stuff, so this seems like the correct behavior for Google. Why should my thin pages rank below known scraper sites (like Pipl) that scrape my content, or below irrelevant pages (like pages on Wikipedia where the search phrase doesn't exist, the words just appear on the page nowhere near each other)? If I'm the only source of a piece of information that someone searches for, I expect to be #1 for someone searching for it. During my penalty phase I was not. Since October 13 I am again.