\begin{tikzpicture}

	\draw[thick,->] (0,0) -- (3.5,0);
	\draw[thick,->] (0,0) -- (0,3.5);

	\onslide<3->{\draw[densely dotted] (0.0,4.0) -- (4,0);}

	\node at (3.3, -0.2) {$x_1$};
	\node at (-0.2, 3.3) {$x_2$};

	\node at (-0.1, -0.3) {$(0,0)$};
	\node at (-0.1, 2.3) {$(0,1)$};
	\node at (2.0, -0.3) {$(1,0)$};
	\node at (2.2, 2.3) {$(1,1)$};
	\onslide<3->{\node at (2.7, 1.0) {$x_1 + x_2 = \theta = 2$};}

	\filldraw (0,0) circle (2pt);
	\filldraw (0,2) circle (2pt);
	\filldraw (2,0) circle (2pt);
	\filldraw (2,2) circle (2pt);
\end{tikzpicture}