Luminosity-metallicity relation for LCGs from SDSS DR7 (Izotov et al. 2011a) (small blue circles). The location of HG 031203 is shown by the large filled red star. By filled squares are denoted the three most metal-poor BCDs (SBS 0335–052E, SBS 0335–052W and I Zw 18) (Guseva et al. 2009), the intermediate-redshift (z  <  1) extremely low-metallicity emission-line galaxies studied by Kakazu et al. (2007), and luminous metal-poor star-forming galaxies at z  ~  0.7 studied by Hoyos et al. (2005). The position of the Lyman-break galaxies at z  ~  3 of Pettini et al. (2001) is marked as LBG with a dotted-line rectangle in the upper right. The emission-line galaxies studied by Guseva et al. (2009) are shown by dots (red dots are data from our observations and black dots are SDSS data). The best linear-likelihood fit to the strongly star-forming galaxies derived by Izotov et al. (2011a) is shown by the black straight line. The fit to emission-line galaxies studied by Guseva et al. (2009) is shown by the red line. Local (z < 0.2) HGs compiled by Margutti et al. (2007) are shown by filled red circles. Data from Levesque et al. (2010a), Hammer et al. (2006) and Han et al. (2010) are shown by green, light blue and purple circles, respectively. (A colour version of this figure is available in the online journal.)