I suspect your cut was not wavy but the plaid print is not on the straight of grain or printed straight (a common problem) which gave the illusion of your cut being wavy. Try tugging your fabric on the two corners diagonal from each other (on the bias) to straighten things up. This will help if it is a woven plaid. However if it is a print there isn't much you can do unless you painstakingly try to line up your rule with the plaid design, but if is way off you may end cutting partially on the bias.
Depending on how small you will be cutting down your plaid, it may not be necessary as the smaller units will make the wavy illusion less obvious.