I mean that the Gyro signal (angle variation, deg/s) can be seen as the first derivative of the ACC signal (deg)
Gyro signal should be zero when ACC signal is horizontal (dX/dt=0)
On the first diagram the red curve cross the zero axis when the black curve reaches a max or a min.
in my opinion, if the gyro signal is not zero, which means the Angle wil go on changing, the ACC signal will not be the max or min.
And i wil go on modifying the parameter to improve my kalmanfitering.