Data distributions of the profiles of the spectral lines J = 5–4 and J = 2–1 of HCN and HNC parametrised with the variables (I, Δv). The upper and lower rows show the data distributions of J = 5–4 and J = 2–1, respectively, for HCN (left panels) and HNC (right panels). The HNC/HCN ratio and the source’s size are fixed to 0.8 and 5 arcsec, respectively. As the data are distributed by branches of points, corresponding to the steps of ΔV_sys, only those with ΔV_sys = 1, 5, 10 and 15 km/s are showcased for the sake of clarity.