A question on Pendulum experiment

[Jens Noriʇzsɔɥ]

and the graph is exactly as you explained it should be!  One can very clearly see the frequencies "grid" - although I  see them spaced every ~0.005Hz

Please note that the grid “effect” is in the period. The observed frequencies are not strictly equidistant.

I suppose the 'extra' data points I'm seeing on my Samsung Galaxy S6 Active phone are due to some inaccuracies of that device sensor?

They occur at lower oscillation amplitudes, so apparently at the end of the measurement. An unstable pendulum could be a reason, some torsion, for instance. The algorithm utilizes a simple sum of the gyroscope axes (giving flexibility in the smartphone’s orientation)