Random points in a rectangle

Question

I have made a graph as shown in the following figure:

For each rectangle I'd like to generate n random points in it, is it possible?

My code:

\begin{tikzpicture}
\begin{axis}[
    grid=major,
    width=\textwidth,
    height=\axisdefaultheight,
    xlabel = Time $(s)$,
    ylabel = Frequency,
    xmin = 0, xmax = 13,
    ymin = 0, ymax = 4000,
]

    \draw[pattern=north west lines, pattern color=blue, line width=1pt] (0,0) rectangle (40,100);
    \draw[pattern=north east lines, pattern color=blue, line width=1pt] (40,0) rectangle (80,100);
    \draw[pattern=north west lines, pattern color=blue, line width=1pt] (80,0) rectangle (120,100);

    \draw[pattern=north east lines, pattern color=green, line width=1pt] (0,100) rectangle (40,200);
    \draw[pattern=north west lines, pattern color=green, line width=1pt] (40,100) rectangle (80,200);
    \draw[pattern=north east lines, pattern color=green, line width=1pt] (80,100) rectangle (120,200);

    \draw[pattern=north west lines, pattern color=red, line width=1pt] (0,200) rectangle (40,300);
    \draw[pattern=north east lines, pattern color=red, line width=1pt] (40,200) rectangle (80,300);
    \draw[pattern=north west lines, pattern color=red, line width=1pt] (80,200) rectangle (120,300);

    \draw[pattern=north east lines, pattern color=orange, line width=1pt] (0,300) rectangle (40,400);
    \draw[pattern=north west lines, pattern color=orange, line width=1pt] (40,300) rectangle (80,400);
    \draw[pattern=north east lines, pattern color=orange, line width=1pt] (80,300) rectangle (120,400);


\end{axis}
\draw[decoration={brace,mirror,raise=15pt},decorate]   (0,0) -- node[below=16pt] {\textbf{C}} (3.2,0);
\draw[decoration={brace,mirror},decorate]   (10.8,0) -- node[xshift=10pt] {\rotatebox{-90}{\textbf{band}}} (10.8,1.4);
\end{tikzpicture}

Answer 1

EDIT: I realized that my original answer (below) does not work for newer versions of pgfplots (since there is some mismatch between xmax, ymax and the explicit coordinates you are using for the rectangles. I fixed that, and also split the rectangles in smaller pieces. The number of points is controlled by samples=100.

\documentclass[tikz,border=3.14mm]{standalone}
\usetikzlibrary{patterns}
\usepackage{pgfplots}
\pgfplotsset{compat=1.16}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
    grid=major,
    width=\textwidth,
    height=\axisdefaultheight,
    xlabel = Time $(s)$,
    ylabel = Frequency,
    xmin = 0, xmax = 130,
    ymin = 0, ymax = 4000,
]
%
    \draw[pattern=north west lines, pattern color=blue, line width=1pt] (0,00) rectangle (40,1000);
    \draw[pattern=north east lines, pattern color=blue, line width=1pt] (40,00) rectangle (80,1000);
    \draw[pattern=north west lines, pattern color=blue, line width=1pt] (80,00) rectangle (120,1000);
%
    \draw[pattern=north east lines, pattern color=green, line width=1pt] (0,1000) rectangle (40,2000);
    \draw[pattern=north west lines, pattern color=green, line width=1pt] (40,1000) rectangle (80,2000);
    \draw[pattern=north east lines, pattern color=green, line width=1pt] (80,1000) rectangle (120,2000);
%
    \draw[pattern=north west lines, pattern color=red, line width=1pt] (0,2000) rectangle (40,3000);
    \draw[pattern=north east lines, pattern color=red, line width=1pt] (40,2000) rectangle (80,3000);
    \draw[pattern=north west lines, pattern color=red, line width=1pt] (80,2000) rectangle (120,3000);
%
    \draw[pattern=north east lines, pattern color=orange, line width=1pt] (0,3000) rectangle (40,4000);
    \draw[pattern=north west lines, pattern color=orange, line width=1pt] (40,3000) rectangle (80,4000);
    \draw[pattern=north east lines, pattern color=orange, line width=1pt] (80,3000) rectangle (120,4000);
    \pgfplotsinvokeforeach{1,...,12}
    {\pgfmathtruncatemacro{\X}{mod((#1-1),3)}
    \pgfmathtruncatemacro{\Y}{int((#1-1)/3+1)}
    \addplot[scatter,clip=true, clip marker paths=true,
        only marks, mark=*, samples=100,point meta=int(#1)             
    ]({20+40*\X+20*rand},{-500+1000*\Y+500*rand});}
\end{axis}
\draw[decoration={brace,mirror,raise=15pt},decorate]   (0,0) -- node[below=16pt] {\textbf{C}} (3.2,0);
\draw[decoration={brace,mirror},decorate]   (10.8,0) -- node[xshift=10pt] {\rotatebox{-90}{\textbf{band}}} (10.8,1.4);
\end{tikzpicture}
\end{document}

ORIGINAL ANSWER(for older versions of pgfplots, produces your screen shot from your code): I am not quite sure which rectangles you are talking about. This answer takes them to be the large ones, i.e. those which run from x=0 to x=12, but I will be happy to divide them for you. I also took n=100, which is controlled by samples=100.

\documentclass[tikz,border=3.14mm]{standalone}
\usetikzlibrary{patterns}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
    grid=major,
    width=\textwidth,
    height=\axisdefaultheight,
    xlabel = Time $(s)$,
    ylabel = Frequency,
    xmin = 0, xmax = 13,
    ymin = 0, ymax = 4000,
]

    \draw[pattern=north west lines, pattern color=blue, line width=1pt] (0,0) rectangle (40,100);
    \draw[pattern=north east lines, pattern color=blue, line width=1pt] (40,0) rectangle (80,100);
    \draw[pattern=north west lines, pattern color=blue, line width=1pt] (80,0) rectangle (120,100);

    \draw[pattern=north east lines, pattern color=green, line width=1pt] (0,100) rectangle (40,200);
    \draw[pattern=north west lines, pattern color=green, line width=1pt] (40,100) rectangle (80,200);
    \draw[pattern=north east lines, pattern color=green, line width=1pt] (80,100) rectangle (120,200);

    \draw[pattern=north west lines, pattern color=red, line width=1pt] (0,200) rectangle (40,300);
    \draw[pattern=north east lines, pattern color=red, line width=1pt] (40,200) rectangle (80,300);
    \draw[pattern=north west lines, pattern color=red, line width=1pt] (80,200) rectangle (120,300);

    \draw[pattern=north east lines, pattern color=orange, line width=1pt] (0,300) rectangle (40,400);
    \draw[pattern=north west lines, pattern color=orange, line width=1pt] (40,300) rectangle (80,400);
    \draw[pattern=north east lines, pattern color=orange, line width=1pt] (80,300) rectangle (120,400);
    \pgfplotsinvokeforeach{1,2,3,4}
    {\addplot[scatter,clip=true, clip marker paths=true,
        only marks, mark=*, samples=100,point meta=int(y/1000)             
    ]({6+6*rand},{-500+1000*#1+500*rand});}
\end{axis}
\draw[decoration={brace,mirror,raise=15pt},decorate]   (0,0) -- node[below=16pt] {\textbf{C}} (3.2,0);
\draw[decoration={brace,mirror},decorate]   (10.8,0) -- node[xshift=10pt] {\rotatebox{-90}{\textbf{band}}} (10.8,1.4);
\end{tikzpicture}
\end{document}