documentclass[11pt, a4paper, twoside, fleqn, ngerman]{scrreprt}
usepackage{pgfplots}
usepgfplotslibrary{external}
tikzexternalize[shell escape=-enable-write18]
usepackage{tikz}

begin{document}
begin{figure}[ht]
    centering
        input{plot.tikz}
    caption{...}
    label{fig:a}
end{figure}
end{document}

definecolor{mycolor1}{rgb}{0.00000,1.00000,1.00000}% is used for other addplots not shown

begin{tikzpicture}

begin{axis}[%
width=5cm,
height=5cm,
at={(0.758in,0.481in)},
scale only axis,
xmin=-1,
xmax=1,
xlabel style={font=bfseries},
xlabel={Real z},
ymin=-1,
ymax=1,
ylabel style={font=bfseries},
ylabel={Imag z},
axis background/.style={fill=white}
]
addplot [color=black,solid,line width=1.5pt,forget plot]
  table[row sep=crcr]{%
0.52    0.67\
0.82    0.67\
};

addplot [color=black,solid,line width=1.5pt,forget plot]
  table[row sep=crcr]{%
0.52    -0.67\
0.82    -0.67\
};

%% Several other addplot's
end{axis}
end{tikzpicture}%