I have a simple GRC demo where the USRP source goes to a WX GUI FFT sink and WX GUI Scope sink. They appear to be a good tools for sanity checks. One feature I don't find however but I think should be quite necessary is when one needs to examine a portion of the FFT spectrum more closely. But I don't find an option where one can specify the start and stop frequency for the FFT spectrum display. Am I missing something?