Variability of some of the lines observed. The observation dates which correspond to the spectra are shown in the top right corner of each box with its corresponding colour. The spectral resolution is 2 MHz. As discussed in Sect. 6.1, variations of up to a 25% are within the uncertainties of the calibration, pointing and baseline substraction (i.e. left boxes). Variations of intensity higher than a 25% are observed in vibrationally excited lines or masers (i.e. right boxes).