Comparison of 6.7 GHz maser spectra obtained with the Torun telescope. The average spectrum is for 669 observations from MJD 55006 to 60208. The spectra marked by the cyan and red lines are typical for high (H) and low (L) activity states, respectively. The black line represents the normalized χ² values as a measure of variability. The dashed vertical line indicates the systemic velocity (Cesaroni et al. 2005).