documentclass[border=2pt,tikz]{standalone}
usepackage{pgfplots}
pgfplotsset{compat=1.13}

begin{document}
begin{tikzpicture}[font=footnotesize,scale=5]
  pgfmathsetmacrolambda{3}
  pgfmathsetmacroy{9}
  begin{axis}[domain=0:1,ymin=0,ymax=1,samples=500]
    addplot[blue](x,{exp(lambda*(1-x))/(exp(lambda*y*x)+exp(lambda*(1-x)))});
    draw[orange,samples=500,domain=0:1]plot(axis cs:x,{exp(lambda*(1-x))/(exp(lambda*y*x)+exp(lambda*(1-x)))});
  end{axis}
end{tikzpicture}
end{document}