Hexbin plots of the comparison of the Fe XXI (or Fe²⁰⁺) effective collision strengths between three sets (A, B, and D) of present calculations (Υ₁) and Fernández-Menchero et al. (2016, F16, Υ₂) at a relatively low temperature. 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). In data set D, we use the same scaling parameters and the same number of points (four times our default calculation) for the fine energy mesh, as in F16. The darker the color, the greater number of transitions log₁₀(N). The diagonal line in red indicates Υ₁ = Υ₂.