What I have found however is that this does not really limit the IMA usage as much as it seems. Elecrtic assist and gasoline engine power appeared to increase jointly, so accelerating slower, while it engages IMA at a lower power level (only a few "bars"), the accelerating takes longer, so the total amount of battery power used to accelerate to a certain speed tends to be about the same. I climb a fairly steep hill every day to my house (up about 850' ft with stops, then back down 150') and I can climb fast and use lots of bars, or slowly and only a few assist bars, but either way the battery level always drops by the same amount.

If when accelerating you are at an RPM where the IMA is not engaging at all, that probably means you are running below the most efficient engine loading (typically low 2000s RPMs)-- The primary goal of the hybrid system is to allow the engine to run at its most efficient load most of the time, so if you are running below that, it won't offer any assist (doing so would make no sense), and though it may appear worse on the instant MPG, you'd probably be better off accelerating harder -- it will take the same amount of energy to accelerate a car to a given speed, so the goal is to accelerate with the engine in its most efficient RPM/load.

Allowing a car to perform well with a much smaller engine is a major contribution of the hybrid system. The engine's compression ratio and valve timing can also be tuned in such a way that it gains efficiency at the expense of low-end torque. This would comprimise driveability in a normal car, but works great when coupled with an elecrtic motor. Even if you only use that eletric power sparingly, its presence allows you to run a smaller, more efficient engine all the time, so it is still helping--even on flat highway runs. (This is the point critics miss when they claim hybrid systems don't help on the highway)

Another point of note is that the "charge" and "assist" bars are not to scale -- max charging is limited to 50amps, while max assist is 100amps -- judging by the scale it would seem charging far outweight assist but that's not really the case.