Labels an pgfplots

\RequirePackage{filecontents}
\begin{filecontents}{\jobname Data.asc}
2   224694229071385000  8.97    8.72    3.275
5   561735572678462000  8.25    5.25    4.91
10  1123471145356920000 8.5 4.92    4.92
30  3370413436070770000 7.75    3.5 6.19
60  6740826872141550000 7.09    4.5 5.82
120 13481653744283100000    6.85    4.75    6.06
\end{filecontents}
\documentclass{article}
\usepackage{scrlfile}
\usepackage{pgfplots}
\pgfplotsset{compat=1.10}
\usepackage{siunitx}
\usepgfplotslibrary{external}
\tikzexternalize
\tikzsetexternalprefix{external_figs/}

\usepackage{etoolbox}
\makeatletter
\newif\if@lateexternal
\newcommand*{\nextwithlateexternal}{\@lateexternaltrue}
\renewcommand*{\@lateexternalfalse}{\global\let\if@lateexternal\iffalse}
\patchcmd\tikzexternal@externalizefig@systemcall@@
  {\immediate\write18{\pgf@tempa}}%
  {\immediate\write18{\pgf@tempa}%
    \if@lateexternal
      \begingroup
        \protected@edef\reserved@a{%
          \noexpand\endgroup
          \noexpand\AfterReadingMainAux{%
            \noexpand\immediate\noexpand\write18{%
              \expandafter\detokenize\expandafter{\pgf@tempa}}%
          }%
        }%
      \reserved@a
    \fi
  }%
  {}%
  {\patchFailedError}
\apptocmd\tikzexternal@externalizefig@systemcall@@
  {\@lateexternalfalse}
  {}
  {\patchFailedError}
\makeatother

\begin{document}
\nextwithlateexternal% späte Externalisierung für nächstes tikzpicture aktivieren
\begin{tikzpicture}
\begin{axis}[
   xlabel = {xlabel},
   ylabel = {ylabel 1},
   smooth,
   axis y line*=left, 
   xmin = -2e18,
   xmax = 1.6e19,
   scaled x ticks=false,
   xticklabel={\pgfmathprintnumber[sci,sci generic={mantissa sep=\times,exponent={10^{##1}}}]{\tick}},
   xtick = {0,5e18,1e19,1.5e19},   
   legend style={draw=none},
]
\addplot[black, mark =x] table [x index = 1, y index = 2] {\jobname Data.asc};
\label{1}
\addplot[red, mark =x] table [x index = 1, y index = 3] {\jobname Data.asc};
\label{2}
\end{axis}

\begin{axis}[
   ylabel = {ylabel 2},
   smooth,
   axis y line*=right, 
   xmin = -2e18,
   xmax = 1.6e19,
   ymin = 2.9,
   ymax = 7.2,   
   %ytick ={3,4,...,9},
   xmajorticks=false,
   scaled x ticks=false, 
   legend style={draw=none},
]
\addlegendimage{/pgfplots/refstyle=1}\addlegendentry{\scriptsize{name 1}};
\addlegendimage{/pgfplots/refstyle=2}\addlegendentry{\scriptsize{name 2}};
\addplot[green, mark =x] table [x index = 1, y index = 4] {\jobname Data.asc};
\addlegendentry{\scriptsize{name 3}};
\end{axis}
\end{tikzpicture}
\end{document}