Why

% Based on formulas by mikuszefski at https://mathematica.stackexchange.com/a/66564 \documentclass[tikz,border=10pt]{standalone} \usepackage{pgfplots} \pgfplotsset{ width = 7cm, compat = 1.11, /pgf/declare function = { bx(\a,\b,\c,\d,\e) = tanh(\d*(\a-\b)) +…