Hexbin plots of the comparison of the Fe XXI (or Fe²⁰⁺) effective collision strengths between two sets (A and B) of calculations (Υ₁) and Fernández-Menchero et al. (2016, Υ₂) at relatively high temperatures. Data set A is the default of the present work. In data set B, we use the scaling parameters of Fernández-Menchero et al. (2016) (Fig. 3). The darker the color, the greater number of transitions log₁₀(N). The diagonal line in red indicates Υ₁ = Υ₂. The dashed lines in red highlight transitions in the “long island” (see Sect. 4.1 for more details).