PGFPlots und underbrace?

[fünfhaus]

Hier ein Minimalbeispiel:

\documentclass[12pt,paper=a4,ngerman]{scrartcl}
\usepackage{babel}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{pgfplots}
\usepackage{amsmath}
\begin{document}
\begin{figure}[htb]
\centering
\begin{tikzpicture} 
\begin{axis}
[
axis lines=middle,
axis line style={->},
xlabel={x},
ylabel={y},
xmin=-4,xmax=4,
ymin=0,ymax=0.5,
grid=both,
grid style={line width=.1pt, draw=gray!10},
major grid style={line width=.2pt,draw=gray!50},
minor tick num=4,
legend pos = outer north east
]
\addplot
[
very thick,
cyan!50!black,
mark=none, 
smooth
] 
{1/(0.92600*sqrt(2*pi))*exp(-x^2/(2*0.92600^2))};
\addlegendentry{$\mu=0$, $\sigma = 0,926$} 
\draw[red,very thick,dashed] (axis cs:-3.5,0) -- (axis cs:-3.5,0.09);
\draw[red,very thick,dashed] (axis cs:-2.5,0) -- (axis cs:-2.5,0.09);
\end{axis} 
\end{tikzpicture}
\end{figure}
\end{document}

Im Anhang ein Bild, wie es aussehen soll. Wie geht das mit der "underbrace"?