Spectrojack is a programme by Cédric Roux for visualising realtime spectrograms (sonograms) from JACK applications on linux.
This is a small video showing spectrojack in action by sending some input from Pure Data (PD).
You can get the latest version spectrogram here:
As far as I know this is the only truly Open Source JACK-aware programme with realtime sonogram analysis.
NOTE: The audio of this video is (deliberately) not the most pleasing (sine tones, noise, phasors) so you might want to turn the volume down ;)