Classification of Gaia RVS spectra with t-SNE. Panel a: t-SNE maps with perplexity = 50 of the entire RVS sample consisting of 886080 spectra plotted in a 2D-histogram manner. Panel b: same map but split into training (green) and observed (yellow) samples. Panel c: spectra of the observed sample identified as similar to that of the training sample. Panel d: spectra of the observed sample identified as not being similar to that of the training sample. See Sect. 5 for more details.