Region of 2D Space

Edit and compile if you like:

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\documentclass[border=2pt]{standalone}
\usepackage{tikz}
\tikzset{>=latex}
\begin{document}
    
\begin{tikzpicture}
    %\draw[help lines] (0,0) grid (5,5);
    \path[fill=black!20] (0,0) -- (4.5,0) -- (4.5,3) -- (0,3);
    \draw[thick, ->] (-0.2,0) -- (6,0) node [below right] {$x$};
    \node at (0,-0.4) {\scriptsize$x=0$};
    \node at (4.5,-0.4) {\scriptsize$x=l$};
    \draw[thick, ->] (0,-0.2) -- (0,4.5) node [below left] {$t$};
    \node at (-0.6,0) {\scriptsize$t=0$};
    \node at (-0.6,3) {\scriptsize$t=T$};
    \draw[green, very thick] (4.5,0) -- (4.5,3);
    \draw[dashed] (4.5,0) -- (4.5,3);
    \draw[green, very thick] (0,0) -- (0,3);
    \draw[dashed] (0,0) -- (0,3);
    \draw[green, very thick] (0,0) -- (4.5,0);
    \draw[dashed] (0,0) -- (4.5,0);
    \draw[dashed] (0,3) -- (4.5,3);
    \node at (2.25,1.5) {$R$};
\end{tikzpicture}
    
\end{document}
 
 
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