Summary of the observed evolutionary trends. Panel a: Abundances of o-H₂D⁺ (blue symbols) and N₂D⁺ (red symbols) as a function of the evolutionary class defined in Sect. 2.2. Circles and diamonds refer to results from this work and Giannetti et al. (2019), respectively. Panel b: median R_D factors derived for each evolutionary class defined in our sample. Different colours refer to values obtained from LTE and NLTE analysis discussed in Sects. 4.2.1 and 4.2.2.