Joint probability density functions of the brightness temperature in Mg ii k_2v (left), k₃ (center), and k_2r (right) computed in different cases. Horizontal axes are 1D PRD (left and right panels) and 3D CRD (center). Vertical axes are all 3D PRD. The color of the JPDFs displays the logarithm of the number of counts ranging from white (smallest) to black (biggest). The three nested iso-contours encompass regions with 68%, 95%, and 99% of all pixels. The red lines denote x = y. The small side plots show 1D histograms for the horizontal (top) or vertical (right) axes with mean values indicated by short black lines. The Pearson correlation coefficient r is given in an inset in each panel.