I have a 2008 Altima 2.5 SL with 195K miles.
For a few months now, there has been no response from the EVAP system and there have been no EVAP codes.
Live data shows Vapor pressure = 0, commanded evap purge =0.
Mode 6 related tests are incomplete.
Has anyone experienced this? What fixed it?
If it was an electrical open or short wouldn't that pop up a code?
Does this mean it's an ECU issue?

I'm not sure what you mean by "no response" from the evap. The ECM does all the evap testing, and it's either alive and talking to your scanner or it isn't. EONV testing always occurs while the car is parked and only under certain conditions, there has to be a certain amount of temperature drop in the tank after shutdown and the tank needs to be between 1/4~3/4 full. So if you're someone who always tops the tank then the ECM may never initiate an EONV.

What is EONV? Is it Engine On, No Voltage?

These graphs show what I mean by "dead" or "Unresponsive".
Evap pressure and commanded purge are always zero.

Sorry, That commanded purge graph is the way it used to be. Here is the current status.

According to the manual, the pressure sensor doesn't even go to zero so a code should have set.

EONV is "Engine Off Natural Vacuum", it's the abbreviation for the procedure the ECM uses to confirm the fuel system isn't leaking. After the car is shut off the fuel cools and creates a partial vacuum in the tank. The ECM knows the approximate amount of air vs fuel and how much the temperature dropped. So it can calculate how much vacuum it should see, then watch to see if it holds that value for a minimum period. If the tank fails one or the other test, the ECM codes for a leak.

The only time the ECM looks at absolute values for the sensor is during EONV. During driving it only uses it as a rough test to make sure the Purge and Vent Valves are opening and closing properly. With the purge volume constantly being adjusted during driving, the readings from the sensor predictably go all over the place. They don't really mean anything unless they increase or decrease and stay there when they shouldn't, that means something is stuck.

PS - Forgot to mention, the sensor (and sometimes the FTT as well) will often read flatline on a scanner during normal operation simply because the ECM isn't monitoring them when it doesn't need to That may be what you're seeing. Putting a voltmeter on the sensor will give you a surer idea of whether it's working.

Thanks, I'll try to check that next weekend.