Lines detected for a HR (R = 20 000, top) and a LR (R = 6000, bottom) set-up, for a Solar-like star. The names of the elements are indicated on the left-hand side of the plots. The number of identified lines is written in red, next to each element. The points are located at the wavelength where a line is detected. The area of the circles is proportional to the purity of the line, and their colour to the minimum S/N_resol required to detect the line. A simple way to read this plot is the following: if a point is difficult to visualise (small in size and white), then the spectral line is difficult to detect and use. The minimum purity plotted is 0.6. Similarly, lines that require a S/N_resol > 50 in HR or S/N_resol > 100 in LR are excluded. Indicatively, yellow ’+’ symbols are located to the wavelengths at which the Gaia-ESO survey has identified, for R = 40 000, lines that are reliable and pure for spectral synthesis (synflag=‘Y’, see Heiter et al. 2021, and Sect. 4.1).