documentclass[a4paper,11pt]{article}
usepackage{tikz,pgfplots}
usetikzlibrary{fadings}
begin{document}
begin{tikzpicture}[>=stealth]
  begin{axis}[scale=3, domain=-1.1:6.5, xmin=-1, xmax=6.5,ymax=2, axis lines=none]

   addplot+[no marks,red,dashed,domain=-1:6.5] {0.7};
   addplot+[no marks,red,dashed,domain=-1:6.5,path fading=east] {0.8}; %not working


   draw[red,dashed,path fading=east] (axis cs:-1,0.9) -- (axis cs:6.5,0.9); %not working
   draw[red,dashed,path fading=east] (axis cs:-1,0.9) -- (axis cs:6.5,1);

  end{axis}

end{tikzpicture}
end{document}

documentclass[a4paper,11pt]{article}
usepackage{pgfplots}
pgfplotsset{compat=1.7}
usetikzlibrary{fadings}

begin{document}
begin{tikzpicture}[>=stealth]
  begin{axis}[domain=-1.1:6.5, xmin=-1, xmax=6.5,ymax=2, axis lines=none]

   addplot+[no marks,red,dashed,domain=-1:6.5] {0.7};
   addplot+[no marks,red,dashed,domain=-1:6.5,path fading=east,fit fading=false] {0.8}; %now working


   draw[red,dashed,path fading=east] (axis cs:-1,0.9) -- (axis cs:6.5,0.9) ++(0,1pt); %now working
   draw[red,dashed,path fading=east] (axis cs:-1,0.9) -- (axis cs:6.5,1);

  end{axis}

end{tikzpicture}
end{document}