Please, any idea how to fix it?
This graph is 'normal' for the z-axis as the acceleration you are seeing is gravity acting on your quadcopter. Gravity is just under 10m/s^2 depending on where you are in the world, divide the axis scale by 100 and you have 1G acting on your quad during flight. (where 1G is one times the acceleration due to gravity) So yes, your graph should be centered around -1000.
What may not be so normal is the high level of vibration you are are seeing in the z-axis, ideally this should be below 0.2G. (+- 200 on the graph scale)
See Here for a further explanation and graphical representation.
Thanks Christopher!! now I understand. I have to modify the way I've fixed my APM, it's too rigid!!!