looking at the O2 capture trace i'm inclined to think that the rear sensor is shot but i wouldn't rush to that descision without knowing what the time base is in your graphic.
without sounding like a knowitall: what O2 sensors measure is the content of oxigen in the exhaust stream, the less oxigen the higher the voltage output typically 0.9 volt, the more oxigen
the lower the output 0.1 volt the cycling usually 2-5 times per second . based on this, the 2 usptream sensors are working fine ( considering that the time frame is 1 second per division) so is the fuel control.
Now, the rear O2 works just the same as the front ones except this one is used by the computer to monitor catalyst efficiency (based on the convertor's oxigen storage capacity).
the rear sensor should read high when the converter is storing oxigen and low when releasing it. [ok pause...... this is very hard for me to explain because i'm a very slow typer (it has taken me 10 minutes to write this so far) ] i'd like to see the rear sensor hanging around 700mv and switching at a ratio of at least 1:5 . i would also interpret that graph like an exhaust leak between the upstream and downstream sensors.
Ducky, call me i'll
confuse you even more . 
It is a big car but still.
