Möglicherweise ist irgendwie sowas gesucht:
![alt text][1]
Eine Zusatzachse kann man mit TikZ-Mitteln in pgfplots reinschmuggeln.
\documentclass[margin=20mm]{standalone}
%\documentclass{article}
\usepackage[ngerman]{babel}
\usepackage{pgfplots}
\pgfplotsset{compat=1.9}
% Grouping the common style settings here to make the code below easier to read
\pgfkeys{/pgfplots/AxisStyle/.style={
width=13.5cm, height=5cm,
axis x line=center,
axis y line=middle,
samples=100,
ymin=-1, ymax=1.5,
xmin=0.0, xmax=7.0,
ytick={1},
%enlargelimits,
%enlarge x limits=0.05,
enlarge x limits={abs value=0.5,lower},
enlarge y limits=0.05,
axis line style = {-latex},
xlabel=$\varphi$[rad],xlabel style={anchor=north},
ylabel=$y$[\%],ylabel style={anchor=east},
}}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
AxisStyle,
xtick={1.5708, 3.14159, 4.7123889, 6.28318},
xticklabels={$\frac{\pi}{2}$, $\pi$, $\frac{3\pi}{2}$, $2\pi$}
]
\addplot [mark=none,thick, domain=0:2*pi] {sin(deg(x))^2};
% Zusatzachse
\draw[yshift=-0.75cm, -latex] (axis cs: 0,0) -- (axis cs: 7,0) node[below, xshift=-5pt]{$t$[s]};
%
\pgfplotsinvokeforeach{1.5708, 3.14159, 4.7123889, 6.28318}{
\draw [yshift=-0.75cm] (axis cs: #1, 0.07) -- (axis cs: #1,-0.07);
}
%
\foreach \x/\text in {1.5708/$\frac{T}{4}$, 3.14159/$\frac{T}{2}$, 4.7123889/$\frac{3T}{4}$, 6.28318/$T$}{%
\edef\temp{%
\noexpand
\node[yshift=-1.1cm] at (axis cs:\x,-0.07) {\text};
}
\temp
}%
\end{axis}
\end{tikzpicture}
\end{document}
[1]: http://texwelt.de/wissen/upfiles/55555555_62.png
