α_CO vs. 12 + log (O/H) for nearby galaxies. The lower limits for SBS 0335−052 and I Zw 18 are shown as filled blue circles; data from Leroy et al. (2011) and Bolatto et al. (2008) are shown as filled squares and ×, respectively. The steeper regression, indicated by a dashed line, has a slope of −1.3 (Genzel et al. 2012) and the shallower one, shown by a dotted line, a slope of −0.9 (Sargent et al. 2013).