Upper panel: ROBIN light curves of BL Lacertae at 24 and 8 GHz (light green and orange dots, respectively) shown together with the re-scaled (see the main text for the details) 20 GHz ones from the F-GAMMA programme (dark green dots). Lower panel: spectral indices calculated using the combined ROBIN and F-GAMMA data at 20-24 GHz and the ROBIN data at 8 GHz. The clear increasing trend of the spectral index (magenta line) reflects the increasing discrepancy between the light curves in the last 2000 days of observations.