Evolution of the amplitude and shape of the periodical signal in the light curve of MAXI J1820+070 between MJD 58283 and MJD 58391 seen in the folded optical data in different sub-intervals. The colour code is the same as in Figs. 1 and 4. The dashed black line corresponds to the best-fitting sinusoid (Eq. (1)) for the entire interval. Except for the first panel, the phase of the signal remains constant, while its shape evolves with time. All the optical bands are in good agreement.