Data points with error bars: observed ratios of the ten spectra (labelled 1–10) to the average spectrum in the representation of Eq. (2) (essentially the spectral ratios with the respective continua divided out and unity-subtracted). Solid green lines: predicted ratio in the representation of Eq. (3) for ionisation component C; dashed blue lines: same for component D; dotted red lines: same for component E; thick magenta line: components A–E combined. The predictions for components A and B are close to zero and not shown here separately.