Dependence of the ion abundance ratio log C⁺⁺(RL)/O⁺⁺(RL) on 12 + log O/H(CEL). Our data are marked by the same symbols as in Fig. 13. Data from Garnett et al. (1995) and García-Rojas & Esteban (2007) are shown by filled and open purple triangles, respectively. Green asterisks are the data from Esteban et al. (2009). Data from Garnett et al. (1999) and Kobulnicky & Skillman (1998) are shown by light blue squares and black circles, respectively. (A color version of this figure is available in the online journal.)