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Hey folks, just released XYscope last month and it's now available within Processing in the Contribution Manager. Details and demo video can be found here.
I still need to make some basic tutorials for hooking up an analog oscilloscope to your computer for using the library, but in the most basic form, you can use a 1/8" to RCA/cinch cable, going from your headphone jack to channels 1 and 2 of the oscilloscope, then switching it to X-Y mode. For best results you'll want to use a DC coupled audio interface.
Make something fancy with it? Please share below!