I have Fing, but it only gives me list of devices connected to the current active wireless network (as well as their services). I can't find any option to scan for networks, let alone see a nice frequency/channel graph...
Buy an old Android phone then. Droid X would be a good choice for that. Because unluess you are jailbroken on an iOS device, Apple no longer allows those types of apps on the market.
Jailbreak iPhone app called "WiFi Analyzer" is probably the closest to what you want. Scans and provides graphs etc. I use it myself on my iPhone4S.
And always worth using Cydia's in built search if you are looking for an app or tweak. It's better at discovering this sort of stuff than you might think!