Stacking regions for J1318-21 reported as blue squares. For the cell size displayed in the figure, STACKER will provide 225 stacked spectra, one for each region. The numbering helps for the bookkeeping. The background grey-scale image is the cube resulting from the preliminary manipulation steps (i.e. masking, shuffling and re-gridding) collapsed along the spectral axis and the black contours enclose the SoFiA-2 mask denote with a yellow colour.