documentclass[varwidth=false, border=2pt]{standalone}
usepackage[utf8]{inputenc} % this is needed for umlauts
usepackage[ngerman]{babel} % this is needed for umlauts
usepackage[T1]{fontenc}    % this is needed for correct output of umlauts in pdf
usepackage[margin=2.5cm]{geometry} %layout

usepackage{pgfplots}
usepgfplotslibrary{dateplot}
usetikzlibrary{pgfplots.dateplot}

begin{document}
begin{tikzpicture}
    begin{axis}[
            date coordinates in=x,
            xticklabel={year},
            x tick label style={align=center, rotate=45},
            yticklabel=$pgfmathprintnumber{tick}$,%,
            date ZERO=1946-06-30,
            xmin={1946-01-01},
            xmax={2010-01-01},
            extra y ticks={5},
            /pgfplots/ytick={0,10,...,100},
            ymin=0, ymax=100,
            width=15cm, height=8cm,     % size of the image
            grid = major,
            grid style={dashed, gray!30},
            legend style={at={(1.15,1)}, anchor=north}
         ]
          addplot[blue, dashed, mark=triangle*] table [x=Wahltag, y=Wahlbeteiligung, col sep=comma] {landtagswahlen-in-bayern.csv};
          addplot[black,mark=square*] table [x=Wahltag, y=CSU, col sep=comma] {landtagswahlen-in-bayern.csv};
          addplot[red,mark=square*] table [x=Wahltag, y=SPD, col sep=comma] {landtagswahlen-in-bayern.csv};
          addplot[green,mark=square*] table [x=Wahltag, y=GRÜNE, col sep=comma] {landtagswahlen-in-bayern.csv};
          addplot[yellow,mark=square*] table [x=Wahltag, y=FDP, col sep=comma] {landtagswahlen-in-bayern.csv};
          legend{Wahlbeteiligung,CSU,SPD,Grüne,FDP}
    end{axis}
end{tikzpicture}
end{document}

documentclass[varwidth=false, border=2pt]{standalone}
usepackage[utf8]{inputenc} % this is needed for umlauts
usepackage[ngerman]{babel} % this is needed for umlauts
usepackage[T1]{fontenc}    % this is needed for correct output of umlauts in pdf
usepackage[margin=2.5cm]{geometry} %layout

usepackage{pgfplots}
usepgfplotslibrary{dateplot}
usetikzlibrary{pgfplots.dateplot}

begin{document}
begin{tikzpicture}
    begin{axis}[
            date coordinates in=x,
            xticklabel={year},
            x tick label style={align=center, rotate=45},
            yticklabel=$pgfmathprintnumber{tick}$,%,
            date ZERO=1946-06-30,
            xtick={1946-01-01,1950-01-01,1954-01-01,1958-01-01,1962-01-01,1966-01-01,1970-01-01,1974-01-01,
                       1978-01-01,1982-01-01,1986-01-01,1990-01-01,1994-01-01,1998-01-01,2002-01-01,2006-01-01,2010-01-01},
            xmin={1946-01-01},
            xmax={2010-01-01},
            extra y ticks={5},
            /pgfplots/ytick={0,10,...,100},
            ymin=0, ymax=100,
            width=15cm, height=8cm,     % size of the image
            grid = major,
            grid style={dashed, gray!30},
            legend style={at={(1.15,1)}, anchor=north}
         ]
%    addplot[blue, dashed, mark=triangle*] table [x=Wahltag, y=Wahlbeteiligung, col sep=comma] {landtagswahlen-in-bayern.csv};
%    addplot[black,mark=square*] table [x=Wahltag, y=CSU, col sep=comma] {landtagswahlen-in-bayern.csv};
%    addplot[red,mark=square*] table [x=Wahltag, y=SPD, col sep=comma] {landtagswahlen-in-bayern.csv};
%    addplot[green,mark=square*] table [x=Wahltag, y=GRÜNE, col sep=comma] {landtagswahlen-in-bayern.csv};
%    addplot[yellow,mark=square*] table [x=Wahltag, y=FDP, col sep=comma] {landtagswahlen-in-bayern.csv};
%    legend{Wahlbeteiligung,CSU,SPD,Grüne,FDP}
    end{axis}
end{tikzpicture}
end{document}

Package pgfplots notification 'compat/show suggested version=true': document ha
s been generated with the most recent feature set (pgfplotsset{compat=1.10}).

PGFPlots: reading {landtagswahlen-in-bayern.csv}
! Undefined control sequence.
GenericError  ...
                                                    #4  errhelp @err@     ...
l.32 ...sep=comma] {landtagswahlen-in-bayern.csv};


The control sequence at the end of the top line
of your error message was never def'ed. If you have
misspelled it (e.g., `hobx'), type `I' and the correct
spelling (e.g., `Ihbox'). Otherwise just continue,
and I'll forget about whatever was undefined.