Graphe d'une fonction

\usepackage{tikz}    %  à placer dans le préambule

\begin{tikzpicture}
\draw [->] (-4,0)  -- (6,0) node [below left] {\(s\)} ;
\draw [->] (0,-1)  -- (0,2.3) node [below right] {\(S_{k+\sigma(s)}\)} ;
\draw [line width = 2pt] (-4,1) -- (0,1) node [above left] {\(1\)} 
                          -- (1.8,1) -- (4,0) node [below] {\(k+\sigma\)} -- (6,0) ;
\draw [dashed] (1.8,0) node [below] {\(k\)} -- (1.8,1) ;
\end{tikzpicture}