How to produce a diagram with dashed circle in between the nodes and arrows?

Question

I want to draw a diagram that just that looks as follows:

where I want the diamond to put side-by-side and I can write something at the left of the diagram with line breaks.

The working that I managed to do is as follows:

\documentclass[paper=a4]{scrbook}
\usepackage{longtable, pdflscape, tikz, tikz-cd} 
\usetikzlibrary{arrows, matrix, positioning, shapes, shapes.geometric, calc, intersections, decorations.pathreplacing}
\newcommand{\tikznode}[2]{\relax
    \ifmmode%
    \tikz[remember picture,baseline=(#1.base),inner sep=0pt] \node (#1) {$#2$};
    \else
    \tikz[remember picture,baseline=(#1.base),inner sep=0pt] \node (#1) {#2};%
    \fi}
\begin{document}
\tikzstyle{polygon} = [regular polygon, regular polygon sides=8, draw, fill=blue!20, 
text width=5em, text badly centered, node distance=3cm, inner sep=0pt]
\tikzstyle{block} = [rectangle, draw, fill=blue!10, 
text width=6em, text centered, rounded corners, minimum height=4em]
\tikzstyle{block2} = [rectangle, draw, fill=blue!20, 
text width=12em, text centered, rounded corners, minimum height=4em]
\tikzstyle{block3} = [rectangle, draw, fill=blue!30, 
text width=12em, text centered, rounded corners, minimum height=4em]
\tikzstyle{block4} = [rectangle, draw, fill=blue!40, 
text width=12em, text centered, rounded corners, minimum height=4em]
\tikzstyle{block5} = [rectangle, draw, fill=blue!50, 
text width=12em, text centered, rounded corners, minimum height=4em]
\tikzstyle{block6} = [rectangle, draw, fill=blue!60, 
text width=12em, text centered, rounded corners, minimum height=4em]
\tikzstyle{line} = [draw, -latex']
\tikzstyle{cloud} = [draw, ellipse,fill=red!20, node distance=2cm,
minimum height=4em]
\begin{figure}[!h]
    \centering
    \begin{tikzpicture}[node distance = 2cm, auto]
        % Place nodes
        \node [cloud] (record) {Recording the EEG signals};
        \node [cloud, below of=record, node distance=3cm] (flattening) {Flattening the recorded EEG signals};
        \node [block2, below of=flattening, node distance=3cm] (semigroup) {Semigroup of EEG signals};
        \node [block2, below of=semigroup, right of=semigroup, node distance=3.5cm] (semigroupdiagonal) {Semigroup of\ diagonal EEG signals};
        \node [block2, below of=semigroup, left of=semigroup, node distance=3.5cm] (semigroupaffine) {Semigroup of affine scaling maps};
        \node [polygon, below of=semigroupaffine, node distance=4cm] (unipotent) {Unipotent matrix of EEG signals};
        \node [polygon, below of=semigroupdiagonal, node distance=4cm] (diagonal) {Diagonal matrix of EEG signals};
    \path [line] (record) -- (flattening);
    \path [line] (flattening) -- (semigroup);
    \path [line] (semigroup) -|  (semigroupaffine);
    \path [line] (semigroup) -|  (semigroupdiagonal);
    \path [line] (semigroupaffine) -- (unipotent);
    \path [line] (semigroupdiagonal) -- (diagonal);

    \coordinate (tmp1) at ($ (record.north)!-0.1!(flattening.north) $);
    \draw[dashed] ([xshift=-7cm]tmp1)node[below right]{Flat EEG} -- ([xshift=7cm]tmp1);
    \coordinate (tmp2) at ($ (flattening.south)!.5!(semigroup.north) $);
    \draw[dashed] ([xshift=-7cm]tmp2)node[below right]{Krohn-Rhodes decomposition of EEG signals} -- ([xshift=7cm]tmp2);

\end{tikzpicture}

\end{figure}
\end{document} 

that produces the following:

How can I achieve this? Please help.

Answer 1

I recommend using positioning instead of just loading it. And use fit for the dashed ellipse. And do not use \tikzstyle.

\documentclass[tikz,border=3mm]{standalone}
\usetikzlibrary{arrows.meta, positioning, shapes, fit}
\tikzset{polygon/.style={regular polygon, regular polygon sides=8, draw, fill=blue!20, 
text width=5em, text badly centered, node distance=3cm, inner sep=0pt},
block/.style={rectangle, draw, fill=blue!10, 
text width=6em, text centered, rounded corners, minimum height=4em},
block2/.style={rectangle, draw, fill=blue!20, 
text width=12em, text centered, rounded corners, minimum height=4em},
block3/.style={rectangle, draw, fill=blue!30, 
text width=12em, text centered, rounded corners, minimum height=4em},
block4/.style={rectangle, draw, fill=blue!40, 
text width=12em, text centered, rounded corners, minimum height=4em},
block5/.style={rectangle, draw, fill=blue!50, 
text width=12em, text centered, rounded corners, minimum height=4em},
block6/.style={rectangle, draw, fill=blue!60, 
text width=12em, text centered, rounded corners, minimum height=4em},
line/.style={draw, -Latex},
cloud/.style={draw, ellipse,fill=red!20, 
minimum height=4em},
rr/.style={draw, rounded rectangle,fill=blue!30,align=center, 
minimum height=4em,minimum width=18em}}
\begin{document}
\begin{tikzpicture}[node distance=1.5cm and 2cm,font=\sffamily,auto,
    hv/.style={to path={-| (\tikztotarget)}},
    vh/.style={to path={|-(\tikztotarget)}},
    ]
    % Place nodes
    \node [cloud] (record) {Recording the EEG signals};
    \node [cloud, below=of record, node distance=3cm] (flattening) {Flattening the recorded EEG signals};
    \node [block2, below=of flattening] (semigroup) {Semigroup of EEG signals};
    \node [block2, below left=of semigroup] (semigroupaffine) {Semigroup of affine scaling maps};
    \node [block2, below right=of semigroup] (semigroupdiagonal) {Semigroup of\ diagonal EEG signals};
    \path (semigroupaffine.south) -- (semigroupdiagonal.south) coordinate[midway,below=2cm] (aux);
    %
    \path (aux) node[left,polygon] (unipotent) {Unipotent matrix of EEG signals}
    node [right,polygon] (diagonal) {Diagonal matrix of EEG signals}
    (aux|-diagonal.south) coordinatebelow=2cm
    (aux2) node[left=1em,rr] (identity) {Semisimple (identity matrix)\ $+$\ nilpotent} 
    node[right=1em,rr] (0-matrix) {Semisimple \ $+$\ nilpotent (0 matrix)} ;
    \path[every pin edge/.style={Latex-,red,thick},pin distance=2em] 
      nodeellipse,draw=red,dashed,fit=(unipotent)(diagonal),
        pin={[align=left,anchor=west]-14:{Unique\
        factorization\dots}}{};
    \path[every edge/.append style={line}] (record) edge (flattening)
        (flattening) edge (semigroup)
        (semigroup) edge[hv]  (semigroupaffine) 
        edge[hv]  (semigroupdiagonal)
        (semigroupaffine) edge[vh] (unipotent)
        (semigroupdiagonal) edge[vh] (diagonal)
        (unipotent) edge (identity)
        (diagonal) edge (0-matrix);
    \path 
    ([xshift=-2em,yshift=1em]current bounding box.north west) coordinate (TL)
    ([xshift=2em,yshift=-1em]current bounding box.south east) coordinate (BR)
    (flattening) -- coordinate (aux3) (semigroup)
    (unipotent) -- coordinate (aux4) (identity);
    \begin{scope}[node distance=1ex,dashed,nodes={align=left,below right}]
     \draw (BR|-TL) -- (TL) node{Flat EEG};
     \draw (BR|-aux3) -- (TL|-aux3) node{Krohn--Rhodes\
      decomposition\ of EEG signals};
     \draw (BR|-aux4) -- (TL|-aux4) node{Jordan--Chevalley\
     \dots}; 
    \end{scope}
\end{tikzpicture}
\end{document}