[LAU] spectrum analyzer to visualize harmonics

[Federico Bruni]

I'm searching a spectrum analyzer software.
What do you recommend?

I've tried JAAA but it's not much user-friendly and there's no manual.
I'd like to visualize the harmonics produced by a guitar string.
How can I achieve what I see in this screenshot?
http://kokkinizita.linuxaudio.org/linuxaudio/jaaa-pict.html

Any other suggestion is welcome.
Thanks
-- 
Federico