And, something I'd not thought about in the context of this diagram.
Let's imagine we put a full air dam on the front of the car, extending to the road (ie an extreme). Now the pressures we can see in the wake would also occur under the car.
We can see from the length of those wake arrows, that if they were placed under the car and pointed downwards, they would help decrease overall lift but could never counteract it entirely - the pressures aren't of great enough magnitude.
However, with a full length, smooth undertray and diffuser, we can get pressures lower than the wake. How low (ie how long an arrow)? I'd be guessing on this car but I'd say of similar magnitude to the 'up' arrows directly above the rear wheels.