2D scatter plots of variability features of the training set explained in Sect. 3.1. DSCTs: black squares, RRLs: green x, CEPHs: red x, EBs: blue x, T2CEPHs: yellow squares, LPVs: magenta x, QSOs: red squares, BVs: cyan squares, and NonVars: grey crosses. From left to right, clockwise: period versus period S/N, B_E − R_E (i.e., color) versus B_E band magnitude, η versus η^e, and ψ^CS versus Q_{3−1 | B − R}. The contour line shows spatial distribution of about 550k field sources. To generate the contour line, we built a 2D histogram of the field sources and then used the counts in each 2D-bin. The figure shows the two contour levels of 100 (thick gray line) and 1000 (thick black line). The majority of these field sources are probably non-variables. Different variable classes are separately grouped in the 2D space of variability features. We plot only one out of ten samples from the training set for better legibility of the figure. See text for details.