I believe they are called spectrum analyzers, and I highly doubt you can find them as add-ons. They ones included in shelf systems are nothing more than a gimmick to lull the unwary buyer into choosing the most "glittery" shelf system. There are professional pieces of equipment that serve a similar function but are very precise. In your case (I read your other post), one would be prohibitively expensive.
In my opinion, there has been a silly "shelf-system war" going on the past 10 or so years in which each manufacturer tries to out-gimmick the others with more and more blinking lights. If you walk own the shelf-system aisle in any electronics store, you'll see what I mean.
