Temporal variation of the effective geometric scattering cross-section, converted from the measurements of r-band magnitude, of interstellar comet 2I/2019 Q4 (Borisov). The downward trend is noticeable. Data points from the two observatories are discriminated by different point symbols as shown in the legend of the plot in the lower left corner. The gray dashed line is the best linear least-squares fit, whereas the vertical dotted gray line marks the perihelion epoch of the comet.