Radial metallicity gradients for galaxies NGC 0628, NGC 1566, NGC 2835, and NGC 4303. Metallicity estimates are obtained with our, C17, N22 and PG16 calibrations (from left to right) and reported as small dots in each panel. Big diamonds are the direct measurements, not used to calibrate gradients but reported for visualisation purposes. The solid lines represent the linear best fit performed by excluding data at r < 0.5 r_eff, and small black dots are the indirect metallicity median values evaluated in 0.36 r_eff, 0.81 r_eff, 0.46 r_eff, and 0.62 r_eff wide bins, respectively.