begin{filecontents*}{data.txt}
a b d c
9 1 a 2
8 2 b 4
7 3 c 6
6 4 d 8
5 5 e 10
4 6 f 12
3 7 g 14
2 8 x 16
1 9 x 18
end{filecontents*}
documentclass[paper=a4,ngerman,xcolor=dvipsnames]{article}
usepackage[ngerman]{babel}
usepackage[utf8]{inputenc}
usepackage{pgfplots}
pgfplotsset{compat=1.14}
usepackage{siunitx}
usepackage{caption}
usepackage{graphicx}
usepackage{lipsum}

pgfplotscreateplotcyclelist{mycolorlist}{
blue!99!black,dashed,every mark/.append style={fill=blue!10!black},mark=+\
red!99!black,solid,every mark/.append style={fill=blue!10!black},mark=+\
green!99!black,densely dashed,every mark/.append style={fill=blue!10!black},mark=+\
yellow!80!black,every mark/.append style={fill=blue!10!black},mark=+\
}

defshowmark#1{tikznode[red!95!black]{pgfuseplotmark{#1}};}

begin{document}
begin{figure}
  begin{tikzpicture}
    begin{axis}[
        grid=both,
        width=12cm,
        height=9cm,
        axis equal,
        xtick pos=left,
        ytick pos=left,
        title style={draw,fill=none,name=legend},
        title={%
            showmark{pentagon*}=$SI{100}{kiloHz}$,
            showmark{triangle*}=$SI{10}{kiloHz}$,
            showmark{square*}=$SI{1}{kiloHz}$,
            showmark{diamond*}=$SI{100}{Hz}$,
            showmark{10-pointed star}=$SI{10}{Hz}$,
            showmark{oplus}=$SI{1}{Hz}$,
            showmark{o}=$SI{20}{mHz}$},
        xlabel={R}, ylabel={Z},
        legend style={at={(0.02,0.98)},anchor=north west,cells= anchor=west}},
        legend style={font=footnotesize},
        cycle list name=mycolorlist,
        scatter,
        point meta=explicit symbolic,
        scatter/classes={
          x={},% empty argument means: use the default style
          g={mark=pentagon*,red!95!black},
          f={mark=triangle*,red!95!black},
          e={mark=square*,red!95!black},
          d={mark=diamond*,red!95!black},
          c={mark=10-pointed star,red!95!black},
          b={mark=oplus,red!95!black},
          a={mark=o,red!95!black},
          z={mark=*,red!80!black}
          },
        ]
        addplot table [x=b,y=c, meta=d]{data.txt};
        addplot table [x=c,y=b, meta=d]{data.txt};
        addplot table [x=a,y=c, meta=d]{data.txt};
        addplot table [x=b,y=a, meta=d]{data.txt};

        legend{1, 2, 3 ,4}
   end{axis}
  end{tikzpicture}
end{figure}
end{document}