Although you may feel this is blatant plagiarism, there are a few things you should consider before pursuing this in any legal setting.
How similar is the design to yours, as-in, is it unmistakably similar or identical to a non-bias 3rd party? Is there a possibility your design could be considered generic enough to not be copyrightable in a legal sense? What is the likelihood this company brought in a 3rd party designer and instructed them to design something with ink and pen, of which a new and unique piece would have been created without infringement. Did you leave them with samples of your work from-which you might be able to claim they copied (you said it was over a year later that this tshirt was introduced). How many sales have they made for this design? 10, 100, 1000? All are small relatively when compared to the dollar amount you will have to front to bring suit. How large is this company? (you can't sue someone for a million dollars if they don't have it).
Any of these scenarios (and more) might render your case sunk. Unfortunately you won't be able to figure some of these out without moving forward.
In the event you do decide to move forward, some general advice:
Lawyers are not cheap (obviously), and they will do whatever you pay them to do -- which is not always what is best to do. A good lawyer might advise you before plotting ahead, however you might easily find a lawyer who will sue for something that is blatantly not a case -- effectively eating up your money for nothing.
If you do not already have a preferred lawyer, your local BAR should be able to make some recommendations. Most have a referral program where for a small fee, they will provide recommendations to cheap or free consultations. You do not need to take the first recommendation! You can have several until you find a lawyer that you feel conformable with. Although not required, I would advise to find one with some previous domain-relevant experience.
There will be a discovery period in which both parties get to request and examine evidence from which they will build a case. They will get to see everything you have on them, and you the same. Unfortunately at this point, you have already filed the claim and if you uncover you do not have a strong case, it may be difficult to back out without taking some loss.
If your claim is under $10,000, then it is likely headed to Small Claims court, instead of Civil court. Civil court may take months to get a court date, whereas small claims court you generally will get a date in a few weeks up to a month. You have the option to file in either (but not both), but know that both carry the same weight in handing down judgement. In both, you are responsible for collection of any winnings rewarded to you, basically all you win is the legal right to attempt collection. So even if you do win, there is no guarantee that you will ever actually see your payment, and worse, you may spend a lot attempting to collect.
Also know that most of these things get settled out of court (to keep official things off the books). This may mean you end up taking 1/2 of what your initial claim was (so start high to give wiggle room - it leaves room for negotiation, and give the judge room to compromise without sinking your case and pocketbooks). If you move forward, your lawyer will likely start off with a threatening letter, and step it up from there. Sometimes even this initial letter is enough to bring a settlement (nobody likes getting angry lawyer letters certified mail first thing in the morning).
So, my recommendation, if you feel very strongly about this, and knowing full well you will burn whatever bridge might be left between you and this company, is to start off with a cheap or free consultation. See what the lawyer says -- go home, and maybe try to get another consultation if possible (guard yourself against hearing only what you want to hear). If both professional opinions make a strong enough case to move forward, and financially it makes sense, go for it.
Disclaimer: I'm not a lawyer. I've had my fair bouts, but by no means should my advice be used as the sole basis for any action. Seek counsel if you feel strongly about your situation.