Comparison of diagnostic lines for four stars. The stars (Sk−67 ° 107, Sk−68 ° 41,Sk−69 ° 52,RMC-109) are organised from high (left) to low (right) effective temperature (in spectral type: O8.5 II, B0.7 Ia, B2 Ia, B5 Ia). The lines are selected due to their roles as good diagnostics for stellar or wind parameters. From top to bottom the lines are sorted by wavelength. We note that the line names here correspond to the names in Table B.1. The green line shows the best fit and the green shaded region shows the 1-σ uncertainty interval.