Projection onto the first two diffusion map coordinates of the set of 995 spectra defined in Sect. A.3. The colour code reflects the temperatures assigned according to their spectral types (filled circles) or literature values (empty circles). Black dots represent the entire set of sources used to construct the diffusion map. The inset plot is a zoom of the problematic zone around 2000 K where temperature variations causes only very subtle changes in the RP spectrum.