Observed elemental abundances [A/Fe] as a function of the observed [Ce/Fe]. The colouring corresponds to the 2D KDE of the 180 points shown. The linear fit was performed with ODR. Note that if we refer to [A/Fe] vs [B/Fe], but plot [B/Fe] vs [A/Fe], the sign of the relationship (correlation or anti-correlation) remains the same. In fact, the swap of the two axes results in a reflection around a diagonal line with a slope of 1. The resulting slope is therefore 1/a, while r_S is the same in both cases.