Top: G-band light curve of a representative variable AGN (Gaia DR3 source_id=4768409993534612992). Bottom: SF obtained with linear (red diamonds) and logarithmic (grey squares) time-lag sampling, and with the smoothed variogram (green plus signs). The legend lists the SF slopes obtained with the four methods described in the text (black) and the final slope (blue) with its standard deviation in brackets. A line with this slope is drawn in blue.