Spectral sequence of the type Ic-BL SN SN2007D. The observed spectra (grey) and smoothed spectra (multiple colours) are plotted as solid lines. Also shown are the determined minimum wavelengths of the Fe II (blue) and Si II (green) features, as well as the corresponding uncertainty on the minimum wavelength, which is influenced by the S/N of the spectrum. A trend of increasing minimum wavelength with time is visible for Fe II and Si II; this is a direct result of the deceleration of the SN ejecta. This plot was generated for each SN in the sample and was used to verify the feature selection and to classify nebular spectra. The last three spectra were classified as nebular, as they have a relatively flat continuum and a strong emission feature near 6300 Å. These spectra were obtained from WISeREP and come from Modjaz et al. (2014), Shivvers et al. (2019).