A small utility that captures stereo audio from an ALSA device and draws a vectorscope - left and right channels on the x and y axis. The screenshot below shows it drawing some nice curves as a test. I made it for working with music in my public domain collection - it helps to identify which recordings are mono and so should be encoded as such.
I don't usually write graphical applications under linux. It calls Xlib directly. Crude, but effective. Click the window to enlarge the plot, it'll auto-shrink to fit the signal amplitude. The parameters are all hard-coded. Compile with 'gcc vectorscope.c -lasound -o vectorscope -lX11 -lpthread'
I wrote this to be 'good enough' to solve a very specific situation. It's a bit buggy, and not at all portable - linux-only, it calls Xlib directly. Released as source only, so you'll need the ALSA and X dev-headers installed to compile.