Reflectivity data were collected over a much wider energy range than displayed in the figure and were normalised to the reflectivity of an aluminium mirror measured under the same polarisation conditions at every spectrum. The reflectance spectra measured with the mirror at each polarizer angle are in Mirror_40x.zip (files 0deg.csv to 360deg.csv) where the first digits of the name are the polariser angle in degrees. Each file contains wavelength [nm], intensity [arb] pairs. The reflectance spectra of the two ReS2 crystals are in files res2_A_40x.zip and res2_B_40x.zip (files 0deg.csv to 360deg.csv as for the mirror). Data in the figure are calculated by dividing the ReS2 reflectance by the mirror reflectance at the corresponding angle.