Nonlinear graph in the Physical Pendulum

[omicgavp]

We did an experiment using the physical pendulum to measure gravitational acceleration g. A graph is shown in this link:
http://i593.photobucket.com/albums/tt20/omicgavp/measuringggraph.jpg"

A nonlinear (possibly chaotic) trend can be seen in our graph even though we assumed small-amplitude oscillations. This trend has been observed as the pivot point goes near to the centroid. How did this happen? It disagrees with our equation (theory) found in the link! Can this be due to the damping force?

 

[omicgavp]

T=period, I=moment of inertia, M=mass of the system, g=gravitational acceleration, d=distance from pivot point to centroid

 

[netheril96]

I guess when d decreases, the otherwise small-amplitude oscillations become not so small, and that is why the graph is no longer linear.