Spectral overview of the N₂D⁺ (top) and N₂H⁺ (bottom) lines observed in the entire sample. In each panel, the source name and evolutionary stage are shown in the lower centre and upper right corners, respectively. The MCWeeds models are shown in red for N₂D⁺ and in blue for N₂H⁺. The dotted lines represent the 3σ noise levels. The vertical dashed lines indicate the V_lsr of the sources (see Table A.2). Spectra have been multiplied by an arbitrary factor, shown in each panel.