How to put a brace and a title?

How to put a brace and a title?

I would like to put a brace between the Q_s and Q_d of the following diagram
    \begin{tikzpicture}[domain=0:5,scale=1,thick]
\usetikzlibrary{calc}   %allows coordinate calculations.

%Define linear parameters for supply and demand
\def\dint{4.5}          %Y-intercept for DEMAND.
\def\dslp{-0.5}         %Slope for DEMAND.
\def\sint{1.2}          %Y-intercept for SUPPLY.
\def\sslp{0.8}          %Slope for SUPPLY.

\def\pfc{2.5}           %Price floor or ceiling

\def\demand{\x,{\dslp*\x+\dint}}
\def\supply{\x,{\sslp*\x+\sint}}

% Define coordinates.
    \coordinate (ints) at ({(\sint-\dint)/(\dslp-\sslp)},{(\sint-\dint)/(\dslp-\sslp)*\sslp+\sint});
    \coordinate (ep) at  (0,{(\sint-\dint)/(\dslp-\sslp)*\sslp+\sint});
    \coordinate (eq) at  ({(\sint-\dint)/(\dslp-\sslp)},0);
    \coordinate (dint) at (0,{\dint});
    \coordinate (sint) at (0,{\sint});
    \coordinate (pfq) at  ({(\pfc-\dint)/(\dslp)},0);
    \coordinate (pfp) at  ({(\pfc-\dint)/(\dslp)},{\pfc});
    \coordinate (sfq) at  ({(\pfc-\sint)/(\sslp)},0);
    \coordinate (sfp) at  ({(\pfc-\sint)/(\sslp)},{\pfc});

% DEMAND
    \draw[thick,color=blue] plot (\demand) node[right] {$P(q) = -\frac{1}{2}q+\frac{9}{2}$};

% SUPPLY
    \draw[thick,color=purple] plot (\supply) node[right] {Supply};

% Draw axes, and dotted equilibrium lines.
    \draw[->] (0,0) -- (6.2,0) node[right] {$Q$};
    \draw[->] (0,0) -- (0,6.2) node[above] {$P$};

    %Price floor and ceiling lines
    \draw[dashed,color=black] plot (\x,{\pfc}) node[right] {$P_c$};
    \draw[dashed] (pfp) -- (pfq) node[below] {$Q_d$};
    \draw[dashed] (sfp) -- (sfq) node[below] {$Q_s$};

\draw[->,baseline=5] ($(0,{\pfc})+(-1.5,0.7)$) node[label= left:Price Ceiling] {} -- ($(0,{\pfc})+(-.1,0.1)$);

\end{tikzpicture}
The result of the code above WITHOUT the brace wanted is:

Also I would like to know how to make a title when I'm making the above diagram using
\documentclass[class=minimal,border=0pt]{standalone}