t-SNE maps of different subsets of spectra. Left panel: all spectra in our GIRAFFE data set (dark blue), containing the training set spectra (green). Extreme outlier spectra have been removed. Middle panel: “inner” observed set, defined as the subset of observed spectra that cover the same area in the t-SNE map as the training set spectra. Right panel: spectra in the observed set that do not cover the same area in the map as the training set. This is our “outer” observed set.