2001 VDC/SC One of a Kind
What you are looking for is a drop in correction value. Usually when you have a vacuum leak and you hit the spot where it allows the accelerant you are spraying to enter the intake the AF Correction will drop to a negative value since the accelerant makes the air flow rich in fuel. If the ECM is adding more than 10% during correction, then the trim may only drop to near 0 (zero).
Is the graph jumping around between 0 and 5% at idle? If so, that's normal. Even if it goes to a negative value, as long as it's not over 7-10%, it is normal. The way the fueling works is the ECM constantly switches rich to lean to control the catalytic converter temperature and operation. When an engine is running good and the management is all operational as intended, you will see fuel trim fluctuation at idle in a graph. The LTFT is the average correction made over time. This number will also change when the car is driven since under load the fuel map changes.
In old cars that used carburetors the fueling was constant based on the engine vacuum. With throttle body injection that came around with CARB requirements and catalytic converters, the fueling was still constant but the injector(s) made it easier to manage fuel volume and consistency based on an oxygen sensor feedback. The systems didn't switch back an forth to control the cat temps as much as to maintain a 14.5-14.9:1 ratio to manage hydrocarbons. Delphi came up with the system that is used now to manage the catalytic converter and help it last longer than the old styles. The old styles would overheat and melt causing blockage and they wouldn't last very long, where catalytic converters made in the last 20 years and managed under the present fueling system last for an unknown time frame when the engine performance is kept up to par.