\documentclass[11pt]{article}
\usepackage[T1]{fontenc}
\usepackage{pgfplots}

\pgfplotsset{compat=1.12}  % Wenn nötig, Versionsnummer runter oder auf »newest« setzen

\begin{document}

\begin{figure}
  \begin{tikzpicture}
  \begin{axis}[
  width=14cm,
  height=10cm,
  scale only axis,
  xmin=0,
  xmax=400,
  xlabel={Normalspannung $\sigma$ in [$\frac{kN}{m^2}$]},
  ymin=0,
  ymax=200,
  ylabel={Scherspannung $\tau$ $\frac{kN}{m^2}$},
  axis x line*=bottom, %top,
  axis y line*=left,
  grid=major,
  every axis plot/.append style={
  	line width=1.5pt,
  	mark size=2pt,
  	mark=ball,forget plot,
  	visualization depends on={x \as \myvalue},
  },
  point meta=y,
  nodes near coords={
  	\ifdim \myvalue pt=125pt \pgfmathprintnumber{\pgfplotspointmeta}\fi
  	\ifdim \myvalue pt=250pt \pgfmathprintnumber{\pgfplotspointmeta}\fi
  	\ifdim \myvalue pt=375pt \pgfmathprintnumber{\pgfplotspointmeta}\fi
  },
  nodes near coords style={text=black}
  ]
 \addplot [red] table[restrict expr to domain={\thisrowno{0}}{125:125}] {datenNS.dat};
 \addplot [green] table[restrict expr to domain={\thisrowno{0}}{125:375}] {datenNS.dat};
 \addplot [blue] table[restrict expr to domain={\thisrowno{0}}{375:375}] {datenNS.dat};
  \end{axis}
  \end{tikzpicture}
	\caption{Normal-Scherspannungs - Diagramm}
\end{figure}

\end{document}