Directly viewing the injector pulses..
 

I've built the interface to read the fuel injectors on our 94 Altima to the sound card input of our laptop and found that for two diodes and a resistor it works quite well actually.. I recorded some waveforms with Audacity, a freeware sound editor, and here are a few pics.

This first one is at a 50 mph cruise. Injector is on when the waveform goes negative.. That's just an artifact of the way I happened to wire the interface.

https://i26.tinypic.com/20aq49j.jpg

The next one is a deceleration mode capture.. Fuel isn't shut off all the way but cut to about 1/5 of the normal idle volume.

https://i30.tinypic.com/a4o653.jpg

Here is a capture of a high acceleration mode..

https://i28.tinypic.com/2u7m1yt.jpg

I have more captures if anyone is interested..

Very interesting! It appears you have 'peak and hold' type injectors in the Altima. They get a burst of amps to open the injectors quickly, then the power tapers off so the injector doesn't overheat while still keeping the power high enough to hold the injector open the appropriate ammount of time, then a quick burst of opposite current to close them quickly, which also tapers off to allow the injector solenoid coil some time to cool.

I wonder if the 3 vertical lines in between each pulse is the crank/cam sensor pulses used when the other injectors are firing?

Unfortunately, the system I used is a brilliant kluge, therefore you can't trust the images completely.

The reason for the tapering of the injector pulse is that the sound card input is designed for AC, not DC signals.. A pulsating DC signal, like the injector signals, looks tapered because the sound card input responds only to the AC portion of the signal..

All four injectors are tied together at the positive end, the extra pulses seen between the injector pulses are almost certainly the other three injectors triggering and being slightly coupled to the injector that I'm reading.

https://i29.tinypic.com/17bz2r.jpg

Most cars have a fuel pressure regulator at the fuel rail so pulse width doesn't necessarily translate directly into how much fuel is being burned. You also need to monitor fuel pressure at the injector.

The pressure regulator is designed to keep the pressure at a certain constant level above the intake manifold pressure as that pressure changes with varying loads on the engine.

Pressure across the injector is very close to a constant.. As close as the manufacturers can reasonably keep it.