Why

\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)) + tanh(-1*\e*(\a-\c)); ex(\x,\y,\z) = exp(-1*(\x-\y)^2/\z); ra(\x,\y) = 0.4*(1-0.4*ex(\x,0.8,0.15) +…