\documentclass{standalone}
\usepackage{tikz}


\begin{document}
\begin{tikzpicture}
\draw[thin,->] (0,0)--(4.2,0) node[below]{$\xi'$};
\draw[thin,->] (0,0)--(0,4.2) node[left]{$\eta'$};

\filldraw[fill=lime] (1,1)--(3.5,1)--(3.5,3.5);


\draw (0,0)--(4,4);
\node[rotate=45] at (2,2.3) {$\xi'=\eta'$};
\node[rotate=90] at (3.7,2.3) {$\xi'=\xi$};
\node at (2.3,.7) {$\eta'=\eta$};

\node at (2.7,1.7) {$\bar{\Delta}(\xi,\eta)$};


\fill (3.5,1) circle (.03) node[right]{$(\xi,\eta)$};
\end{tikzpicture}
\end{document}