\documentclass{article}

\usepackage[dvips]{pstricks} 

\usepackage{pst-plot}

\pagestyle{empty}

\begin{document}

	\psset{xunit=2cm,yunit=2cm}

	\begin{pspicture}(-4,-1)(4,3.5) 

		\psline[linecolor=gray](-3.14,2)(3.14,2)
		\psline[linecolor=gray, linestyle=dashed](3.14,0)(3.14,2)
		% dashed = pointillés
		\psline[linecolor=gray, linestyle=dashed](-3.14,0)(-3.14,2)

		\psplot[plotpoints=200,linecolor=red, linewidth=1pt]{-2.5}{2.5}{x 2 exp 2 div}
		% il faut utiliser la notation polonaise inversée pour tracer des courbes de fonctions
		% 200 points sont  calculés pour tracer la courbe.

		\psaxes[linecolor=black, labels=none, ticks=none]{->}(0,0)(-3.5,-0.5)(3.5,3.5)
		% tracé des axes

		\psline[arrows=C-C, linecolor=black](3.14,-3pt)(3.14,3pt)
		% tracé de marques sur l'axe
		\psline[arrows=C-C, linecolor=black](-3.14,-3pt)(-3.14,3pt)
		\psline[arrows=C-C, linecolor=black](0.345,-3pt)(0.345,3pt)
		\psline[arrows=C-C, linecolor=black](-0.345,-3pt)(-0.345,3pt)

		\uput{8pt}[-90](3.14,0){$\pi$}
		% affichage de la lettre pi
		\uput{8pt}[-90](-3.14,0){$-\pi$}

		\uput[-90](3.5,0){$\theta$}
		\uput[-90](0.345,0){\color{red}$\theta_c$}
		\uput[-90](-0.345,0){\color{red}$-\theta_c$}

		\uput[-180](0,3.5){\color{blue}$E_p$}

		\uput[135](0,2){$2mgl$}
		\uput[-135](0,0){$0$}
		\uput[0](2.5,3.125){\color{red}$mgl\cdot \theta^2/2$}

		\psplot[plotpoints=200,linecolor=blue]{-3.14}{3.14}{1 57.29582 x mul cos sub} 

	\end{pspicture}

\end{document}