WW;
Interesting display, it is the same type waveform that I used to see on an old surplus Bendix aircraft ignition analyzer that I bought years ago. It came with a manual that explained the waveforms so I learned something about what it was telling me.
The initial voltage spike happens before the plug fires, once the gas (fuel vapor/air) between the electrodes is ionized, which takes a few microseconds, the spark jumps the gap and conducts current. This causes the voltage to fall and it is fairly constant until the spark goes out and the gas is de-ionized. Since there is no current through the plug the voltage jumps up and then decays until the next firing.
A high pressure causes the gas to be harder to ionize so it takes a higher voltage as you pointed out. Once the plug fires it shows that the discharge also occurs at a higher voltage in the pressurized cylinder.
The Bendix manual also showed the effects of a bad condenser in a points ignition. The tail of the waveform showed a large ringing that gradually died out.
A 'scope is a very useful instrument for diagnosing ignition problems.
Regards, Neil Tucson, AZ