I can't recommend a toolkit or a set of widgets because ages ago I came to the conclusion that there's no way to use traditional ones for a good DAW interface,

I can't recommend a toolkit or a set of widgets because ages ago I came to the conclusion that there's no way to use traditional ones for a good DAW interface, and something very specific to the task should be developed, which is a pain in the ass. For spectrograms, I could suggest a look at Jaaa's source. It draws directly using wrappers on X11 libraries and it's fast and efficient also on low end hardware. https://wiki.linuxaudio.org/apps/all/jaaa https://wiki.linuxaudio.org/apps/all/jaaa http://kokkinizita.linuxaudio.org/linuxaudio/ http://kokkinizita.linuxaudio.org/linuxaudio/