\documentclass{standalone}

\usepackage{tikz}
\begin{document}


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

    \draw [blue] plot[id=curve, raw gnuplot] function{
      f(x,y) = sin (4*x) * sin (1*y)+  0 * sin (4*y) * sin (1*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}
