\documentclass{standalone}
\usepackage{tikz}


\begin{document}
\begin{tikzpicture}
\fill[fill=lime] (4,0)--(0,0)--(4,4)..controls (3.5,3) and (5,2).. (4,0) ;

\draw[thin,->] (0,0)--(4.2,0) node[below]{$x$};
\draw[thin,->] (0,0)--(0,4.2) node[left]{$t$};

\draw (0,0)--(4,4);
 
\node[rotate=45] at (2,2.3)  {$x=ct$};

\end{tikzpicture}
\end{document}