# Underbracing on a set of points in a plot with pgfplots

I have two sets of points graphed in the same plot. I want to mark them grouped with an under- or overbrazing with a large curly bracket, as I have illustrated by hand here:

I am looking for the same type of brazing as in mathematical formatting with $$\underbrace{a+b+c}_n$$:

\documentclass{article}

\usepackage{pgfplots}
\usepackage{xcolor}

\begin{document}

\begin{tikzpicture}
\pgfplotsset{height=7cm, width=8.5cm}

\begin{axis}[
axis lines = left,
hide x axis,
xmin=0, xmax=8,
ymin=600,ymax=1000
]
(1,700)
(2,700)
(3,700)
(4,700)
(5,700)
};
\end{axis}

\begin{axis}[
hide x axis,
hide y axis,
ymin=600,ymax=1000,
xmin=0, xmax=8
]
(6,900)
(7,900)
};
\node at (axis cs:1, 725) {1};
\node at (axis cs:2, 725) {2};
\node at (axis cs:3, 725) {3};
\node at (axis cs:4, 725) {4};
\node at (axis cs:5, 725) {5};
\node at (axis cs:6, 925) {6};
\node at (axis cs:7, 925) {7};

\end{axis}
\end{tikzpicture}

\end{document}

You can use decorations={brace} to create a brace with tikz between two points.

I defined the required nodes with A, B etc. to reuse the positions.

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{decorations.pathreplacing}

\usepackage{pgfplots}
\usepackage{xcolor}

\begin{document}

\begin{tikzpicture}
\pgfplotsset{height=7cm, width=8.5cm}

\begin{axis}[
axis lines = left,
hide x axis,
xmin=0, xmax=8,
ymin=600,ymax=1000
]
(1,700)
(2,700)
(3,700)
(4,700)
(5,700)
};
\end{axis}

\begin{axis}[
hide x axis,
hide y axis,
ymin=600,ymax=1000,
xmin=0, xmax=8
]
(6,900)
(7,900)
};
\node (A) at (axis cs:1, 725) {1};
\node at (axis cs:2, 725) {2};
\node at (axis cs:3, 725) {3};
\node at (axis cs:4, 725) {4};
\node (B) at (axis cs:5, 725) {5};
\node (C) at (axis cs:6, 925) {6};
\node (D) at (axis cs:7, 925) {7};

\draw [decorate,decoration={brace,amplitude=10pt,mirror,raise=6mm}]
(A.west) -- (B.east) node [black,midway, below, yshift=-10mm] {a};

\draw [decorate,decoration={brace,amplitude=5pt,mirror,raise=6mm}]
(C.west) -- (D.east) node [black,midway, below, yshift=-10mm] {b};

\end{axis}
\end{tikzpicture}

\end{document}