I could not find a simple, free oscilloscope plugin that would run in a 64-bit OSX AU host, so I grabbed an old Apple demo-code project, updated it to the new standards and libraries, and rolled a universal 32/64-bit AU. It works fine for me under 10.9/64 and 10.6/32, and I designated the target as 10.5, so hopefully it works in previous OS versions (not tested).
Other than the updates, the only real change from the Apple code was that I added lines in the UI marking digital 0, -1 and 1 to show both the zero-crossing and where a [1,-1] clamp would flatten the waveform.
There is no trigger function, which I may add in the future if I find the need/time, but there IS a ‘hold’ function for viewing a momentary snapshot of the waveform.
You can grab it here:
(Update: A reader requested the project file, so I’m attaching a link to it here: Updated file link You will probably have to fix the paths to the core audio utility class files (and maybe drag them out and back into the project to get them to behave), and there are a lot of depreciated functions, BUT the code is all there, and the plist and resource file are all set up following the newest AUv2 methods as described by Apple.)