Is there a free realtime spectrograph available for Ubuntu?

This may be a bit overkill, but Baudline is an incredible real-time FFT spectrogram. It's not in Ubuntu repos, so you will need to download it on your own. It displays a real-time analysis of source signal, but it also allows to analyze pre-recorded data. The brightness of a point represent the power of the sound at this frequency, where horizontal axis represents frequencies, and vertical time - so it kind of shows history of your signal.

It's extensive features make it definitely the most advanced spectrogram available for Linux. It may be a bit too complicated for your uses, though. I am not sure what exactly you are looking for.

Jaaa

https://launchpad.net/ubuntu/+source/jaaa/+publishinghistory

The JACK & ALSA Audio Analyser, an audio signal generator and spectrum analyser.

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Glfer

https://launchpad.net/ubuntu/+source/glfer/+publishinghistory

glfer is a GTK+ application with a waterfall spectrum display and slow CW (QRSS) / dual-frequency CW (DFCW) transmitter control interface. [ . . . ] The signal to be analyzed can be acquired in real-time from the sound card or can be read from a WAV file.

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Quisk

https://launchpad.net/ubuntu/+source/quisk/+publishinghistory

The Quisk receiver can read the sample data, tune it, filter it, demodulate it, and send the audio to the sound card for output to external headphones or speakers. The Quisk transmitter can accept microphone input and send that to your transmitter via a soundcard or Ethernet.

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Spek

https://launchpad.net/ubuntu/+source/spek/+publishinghistory

There is also Spek, although as of version 0.8.2 it does not work in realtime. However, there is a feature request and a pull request that may change this.

Spek (IPA: /spɛk/, ‘bacon’ in Dutch) helps to analyse your audio files by showing their spectrogram. Spek is free software available for Unix, Windows and Mac OS X.