Wellenlängenbereich und Achse synchronisieren

\begin{filecontents*}{Nachtsehen.csv}
380     0.000589
390     0.002209
400     0.00929
510     0.997
750     0.00000076
760     0.000000425
770     0.000000241
780     0.000000139
\end{filecontents*}
\begin{filecontents*}{Tagsehen.csv}
380     0
390     0.0001
400     0.0004
550     0.995
560     0.995
750     0.00012
760     0.00006
770     0.00003
780     0.000015
\end{filecontents*}
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{tikz,pgfplots,xcolor}

\pgfplotsinvokeforeach{380,400,450,500,550,600,650,700,750,780}{
  \definecolor{wl#1}{wave}{#1}
}

\pgfdeclarehorizontalshading{spectrum}{0.5cm}{
  color(0cm)=(wl380);
  color(0.2cm)=(wl400); color(0.7cm)=(wl450);
  color(1.2cm)=(wl500); color(1.7cm)=(wl550);
  color(2.2cm)=(wl600); color(2.7cm)=(wl650);
  color(3.2cm)=(wl700); color(3.7cm)=(wl750);
  color(4cm)=(wl780)
}

\begin{document}
\begin{figure}[h]
  \centering
  \begin{tikzpicture}
    \begin{axis}[ymin=0,grid,
      scale only axis,
      width=0.8\textwidth,
      height=0.3\textheight,
      xmin = 380,
      xmax = 780,
      title = {V-$\lambda$-Kurve},
      axis line style={ultra thick},
      xlabel={Wellenlänge $\lambda$ [nm]},
      ylabel={Hellempfindlichkeitsgrad V [-]},
      legend style={at={(1,1)}, anchor=north east, nodes=right},
      name = main,
      ]
      \draw [->] (0,0) -- (5,0) node [midway, below] {Text};
      \addplot [no marks, very thick, red]table {Tagsehen.csv};
      \addplot [no marks, very thick, blue]table {Nachtsehen.csv};
      \legend{Tagsehen, Nachtsehen};
    \end{axis}
    \begin{scope}[shift=(main.below south west)]
      \pgfmathsetmacro\scale{.8\textwidth/4cm}
      \node[inner sep=0pt,below,xscale=\scale] at (.4\textwidth,0) {\pgfuseshading{spectrum}};
    \end{scope}
  \end{tikzpicture}
  \caption {Relative-Spektrale-Hellempfindlichkeitskurve \cite{Reif.2014}}
  \label{Vlambda}
\end{figure}
\end{document}