Well gee it looks like you're rankings have improved more recently and are sticking around up there.. Did they fall again since this screenshot? A true penalty wouldn't allow you to regain ranking like that without it being lifted. I'm assuming you haven't received any notices in Google's Webmaster Tools, correct?
As always, it would probably behoove you to provide the domain for folks here to take a peak at. Although I can't guarantee anyone will be able to provide an answer other than "keep on building SEO for your site!"
Are you targeting other keywords as well that aren't fluctuating? For example "55 gallon trash bags" or "buy trash bags online" or something? (if you aren't targeting those, get on it!) Just trying to get a full picture of the problem.
You do want to vary your anchor text. We understand that is your business and what you sell, but that doesn't mean you can't make your link read "Check out our awesome deal on trash bags by clicking here!" Or "We got trash bags over at http://www.trashbagstore.com" or something along those lines.
That's about all the advice I know to offer at this time. Keep adding more info and I'm sure this community can help you get to the answer. Good luck!