Top: 2D projection (PCA 1 versus PCA 2 coefficients) of the anomalies of the two datasets as divided into four groups each (group 1: blue, group 2: black, group 3: green, group 4: red) in the 20 dimensional PCA hyperspace, by the second k-means clustering applied only on the anomalous quasars. The groups as characterized by further analysis (see Sect. 5) are circled and labelled to reflect the nature of their anomaly. Bottom: mean composite spectrum of each anomaly group for both the datasets. The colors of the spectra correspond to the color of the corresponding group in the top panel.