I am using the following code to construct a timeline:
\usepackage{tikz}
\usetikzlibrary{snakes}
\begin{tikzpicture}
% draw horizontal line
\draw (0,0) -- (2,0);
\draw(2,0) -- (4,0);
\draw (4,0) -- (5,0);
\draw (5,0) -- (7,0);
% draw vertical lines
\foreach \x in {0,1,2,4,5,7}
\draw (\x cm,3pt) -- (\x cm,-3pt);
% draw nodes
% \draw (0,0) node[below=3pt] {$ 0 $} node[above=3pt] {$ $};
\draw (1,0) node[below=3pt] {$ 0 $} node[above=3pt] {$ $};
\draw (2,0) node[below=3pt] {$ t-s $} node[above=3pt] {$ $};
\draw (4,0) node[below=3pt] {$ t $} node[above=3pt] {$ A_t $};
\draw (5,0) node[below=3pt] {$ t+r $} node[above=3pt] {$ R_t $};
\draw (6,0) node[below=3pt] {$ $} node[above=3pt] {$ $};
\draw (7,0) node[below=3pt] {$ ... $} node[above=3pt] {$ $};
\end{tikzpicture}
I want to add a underbrace between t and t+r a "X" below the underbrace. For formulas that goes without any problems:
$\underbrace{(x + 2)^3}_\text{text 1}$
However, for the timeline that won't work. Any ideas how to do this?

