\documentclass{standalone}

\usepackage{tikz}
\begin{document}

 \begin{tikzpicture}[xscale=1,yscale=1]

    \draw [blue] (0,pi/2)--(pi,pi/2);
     \draw [blue] (pi/2,0)--(pi/2,pi);   
     \draw (0,0) rectangle (pi,pi);
  \end{tikzpicture}

 
 
% \begin{tikzpicture}[xscale=1,yscale=1]
%
%    \draw [blue] plot[id=curve, raw gnuplot] function{
%      f(x,y) = sin (3*x) * sin (y)-  1 * sin (3*y) * sin (x);
%      set xrange [0:pi];
%      set yrange [0:pi];
%      set view 0,0;
%      set isosample 3000,3000;
%      %set size square;
%      set cont base;
%      set cntrparam levels incre 0,0.1,0;
%      unset surface;
%      splot f(x,y)
%    };
%    
%     \draw (0,0) rectangle (pi,pi);
%  \end{tikzpicture}

\end{document}
