Platzhalter für fehlende Zahl bei einer Statistik mit pgfplots

\begin{filecontents*}{GDP.csv}
    Jahr,Vereinigte Staaten,United Kingdom,Denmark,Finland,Iceland,Norway,Sweden,Germany,EU28
    1990-01-01,1.9,0.7,1.5,0.7,1.2,1.9,0.8,5.3,..
    1991-01-01,-0.1,-1.1,1.4,-5.9,-0.2,3.1,-1.1,5.1,..
    1992-01-01,3.6,0.4,2.0,-3.3,-3.4,3.6,-1.2,1.9,..
    1993-01-01,2.7,2.5,0.0,-0.7,1.3,2.8,-2.1,-1.0,..
    1994-01-01,4.0,3.9,5.3,3.9,3.6,5.1,4.1,2.5,..
    1995-01-01,2.7,2.5,3.0,4.2,0.1,4.2,4.0,1.7,..
    1996-01-01,3.8,2.5,2.9,3.7,4.8,5.0,1.5,0.8,1.9
    1997-01-01,4.5,4.0,3.3,6.3,5.5,5.3,2.9,1.8,2.9
    1998-01-01,4.4,3.1,2.2,5.4,7.0,2.6,4.2,2.0,3.0
    1999-01-01,4.7,3.2,2.9,4.4,3.9,2.0,4.5,2.0,3.0
    2000-01-01,4.1,3.7,3.7,5.6,4.6,3.2,4.7,3.0,3.8
    2001-01-01,1.0,2.5,0.8,2.6,3.9,2.1,1.6,1.7,2.2
    2002-01-01,1.8,2.5,0.5,1.7,0.6,1.4,2.1,0.0,1.4
    2003-01-01,2.8,3.3,0.4,2.0,2.4,0.9,2.4,-0.7,1.3
    2004-01-01,3.8,2.4,2.7,3.9,8.1,4.0,4.3,1.2,2.5
    2005-01-01,3.3,3.1,2.3,2.8,6.4,2.6,2.8,0.7,2.1
    2006-01-01,2.7,2.5,3.9,4.1,5.0,2.4,4.7,3.7,3.3
    2007-01-01,1.8,2.4,0.9,5.2,9.4,3.0,3.4,3.3,3.0
    2008-01-01,-0.3,-0.5,-0.5,0.7,1.7,0.5,-0.6,1.1,0.4
    2009-01-01,-2.8,-4.2,-4.9,-8.3,-6.5,-1.7,-5.2,-5.6,-4.3
    2010-01-01,2.5,1.7,1.9,3.0,-3.6,0.7,6.0,4.1,2.1
    2011-01-01,1.6,1.5,1.3,2.6,2.0,1.0,2.7,3.7,1.7
    2012-01-01,2.2,1.5,0.2,-1.4,1.3,2.7,-0.3,0.5,-0.4
    2013-01-01,1.7,2.1,0.9,-0.8,4.3,1.0,1.2,0.5,0.3
    2014-01-01,2.4,3.1,1.6,-0.6,2.1,2.0,2.6,1.9,1.8
    2015-01-01,2.6,2.3,1.6,0.0,4.3,2.0,4.5,1.7,2.3
    2016-01-01,...,1.8,2.0,1.9,7.4,1.1,3.2,1.9,1.9
    \end{filecontents*}
    \documentclass[tikz]{standalone}
    \usepackage{pgfplots}
    \usepgfplotslibrary{dateplot}
    \begin{document}
    \sffamily
    \begin{tikzpicture}
        \sffamily
          \begin{axis}[
          date ZERO=1990-01-01,
            date coordinates in=x,
            no marks,
            width=12cm,
            height=6cm,
            enlargelimits=false,
            xticklabel={\tiny\year},
          xtick={1990-01-01,1991-01-01,1992-01-01,1993-01-01,1994-01-01,1995-01-01,1996-01-01,1997-01-01,1998-01-01,1999-01-01,2000-01-01,2001-01-01,2002-01-01,2003-01-01,2004-01-01,2005-01-01,2006-01-01,2007-01-01,2008-01-01,2009-01-01,2010-01-01,2011-01-01,2012-01-01,2013-01-01,2014-01-01,2015-01-01,2016-01-01},
            xtick pos=left,
            xtick align=outside,
            ymin=0,ymax=30,
            ytick={0,4,...,30},
            yticklabel={\pgfmathprintnumber[verbatim]{\tick}\%},
            ytick style={draw=none},
            yticklabel pos=right,
            ymajorgrids=true,
           legend style={
                    draw=none,
                    legend cell align=left,
                    at={(0,1.05)},% <-
                    anchor=south west,% <-
                    legend image code/.code={
                        \filldraw[##1] (-.5ex,-.5ex) rectangle (1ex,1ex);
                    },
                  },
                  legend columns=-1% <-
                  ]   
    \addplot table[col sep=comma,x=Jahr,y={Vereinigte Staaten}] {GDP.csv};
        \addlegendentry{Vereinigte Staaten};
         \addplot table[col sep=comma,x=Jahr,y={United Kingdom}] {GDP.csv};
            \addlegendentry{United Kingdom};
             \addplot table[col sep=comma,x=Jahr,y={Denmark}] {GDP.csv};
                \addlegendentry{Denmark};
                 \addplot table[col sep=comma,x=Jahr,y={Finland}] {GDP.csv};
                    \addlegendentry{Finland};
                     \addplot table[col sep=comma,x=Jahr,y={Iceland}] {GDP.csv};
                        \addlegendentry{Iceland};
                         \addplot table[col sep=comma,x=Jahr,y={Norway}] {GDP.csv};
                            \addlegendentry{Norway};
                             \addplot table[col sep=comma,x=Jahr,y={Sweden}] {GDP.csv};
                                \addlegendentry{Sweden};
                                 \addplot table[col sep=comma,x=Jahr,y={Germany}] {GDP.csv};
                                    \addlegendentry{Germany};
                                     \addplot table[col sep=comma,x=Jahr,y={EU28}] {GDP.csv};
                                        \addlegendentry{EU28};
          \end{axis}
        \end{tikzpicture}
    \end{document}