F_{red 1/0} for Na vs. Ca (upper panel) for Hα vs. Ca (middle panel) and for Hα vs. Na (lower panel) for the season sub-sample. The colour-code indicates the degree of significance: p(F) of the x-axis and of the y-axis below 5% (red), pF of the x-axis below 5% (blue), and p(F) of the y-axis below 5% (purple). The dotted and dashed lines correspond to the soft and strict thresholds (0.2 and 0.5) used in the analysis.