Types of anomalous spectra excluded from this study. The red line shows the trend of flux as a function of wavelength. The shaded regions in each panel show the positions of the anomalies. Top left panel: oscillations. The spectrum shows the repetitive variation of flux about a central value. Top right panel: missing values. These spectra lack values in certain wavelength regions. Bottom left panel: outliers. The spectrum has pronounced peaks or deep troughs. Bottom right panel: negative values. The spectra display negative values at certain wavelengths.