Markierung in pgfplotlegende fehlerhaft übernommen

\documentclass[11pt,a4paper,oneside,chapterprefix,numbers=enddot]{scrbook}

\usepackage{siunitx}                        
\usepackage{graphicx}                       
\usepackage[ngerman]{babel}                 
\usepackage[]{acronym}                      
\usepackage{pgfplots,siunitx}               
\pgfplotsset{width=\textwidth, height = 0.4\textheight,compat=newest, legend style={cells={anchor=west}},every axis legend/.append style={at={(0.99,0.99)},anchor=north east}, ticklabel shift={0.05cm}}
\usepgfplotslibrary{external}
\tikzexternalize[prefix=figures/]{Diplomarbeit}

\begin{document}

\tikzsetnextfilename{BF+WA-sTE+KFsNoLI90}
\begin{figure}[htb]
\centering
\tikzset{every mark/.append style = {scale = 2}}
\begin{tikzpicture}
    \begin{axis}[grid = major, width = \textwidth-45pt, height = 0.4\textheight,
                xlabel near ticks,
                ylabel near ticks,
                xlabel = x,
                ylabel = y1,
                xtick = {1,2,3,4,5},
                ytick = {50,54,58,62,66,70},
                ymin = 50, ymax = 70,
                x tick label style={/pgf/number format/1000 sep=},
                ]
        \addplot[only marks,mark = asterisk]
            coordinates {(1,66.90)(2,55.00)(3,56.18)(4,57.99)(5,55.00)};
        \label{plot1}
    \end{axis}

    \begin{axis}[ylabel near ticks, width = \textwidth-45pt, height = 0.4\textheight,
                yticklabel pos=right,
                ylabel = y2,
                ytick = {0,0.04,0.08,0.12,0.16,0.20},
                ymin=0, ymax=0.20,
                xtick = {1,2,3,4,5},
                x tick label style={/pgf/number format/1000 sep=},
                yticklabel style={/pgf/number format/fixed, /pgf/number format/precision=2},
                legend pos=south east,
                ]
        \addlegendimage{/pgfplots/refstyle=plot1}\addlegendentry{y1}
        \addplot[only marks, mark = triangle]
            coordinates {(1,0.08)(2,0.09)(3,0.09)(4,0.18)(5,0.10)};
        \addlegendentry{y2}
    \end{axis}
\end{tikzpicture}
    \caption{}
    \end{figure}

\end{document}