Minimum difference of the GRB hosts from the ridge-line (illustrated by the dots) of z ~ 0 SDSS star-forming galaxies (top panel). We show [O iii]/Hβ  in black and ([O iii]+[O ii])/Hβ  in red as a function of z in the middle and bottom panel, respectively. In all panels, solid lines denote the most-likely linear regression, with regions of 68% probability in shaded areas. Dusty/normal GRBs are indicated by stars/circles. Error bars are omitted to enhance clarity and replaced by typical errors in the bottom-right corner.