get caption/subcaption every plot using groupplot pgf tikz

Question

Hello I want to have Under each plot (subgroup plot) its caption or label, so I can also reference it in my document. Here in my example I used (a), (b), (c) I can keep them but they are not well aligned, and how to get them referenced in the text. Thanks

\documentclass[french,12pt,oneside,openright]{memoir}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[a4paper]{geometry}
\usepackage[french]{babel} %langue francaise
\usepackage{graphicx,xcolor} %insertion d'images
\usepackage{titlesec}
\graphicspath{{Annexes/images/}} %directory of the images
\usepackage{lipsum} %paragraphes prédéfinis
\usepackage[sonny]{fncychap}
\usepackage{usnomencl}%pour la nomenclature
\usepackage{pgfplots}
\usepackage{titlesec, blindtext, color}
\definecolor{gray75}{gray}{0.75}
\newcommand{\hsp}{\hspace{20pt}}
\titleformat{\chapter}[hang]{\Huge\bfseries}{\thechapter\hsp\textcolor{gray75}{|}\hsp}{0pt}{\Huge\bfseries}
\usepackage{multirow,array}
\usepackage{siunitx}
\usepackage{tocloft} %costumize the list of...
\usepackage{amssymb}
\usepackage{amsmath} %add mathematic symbols
\usepackage{caption}  %for tables andfigures
\usepackage{subcaption} %add many sub (tables/figures) with the same caption
\usepackage{chemfig}
\renewcommand*\printatom[1]{\ensuremath{\mathsf{#1}}}
\usepackage{tikz}
\usepackage{hyperref} %add a hyperlink to the contents
\usepackage{setspace}
\usepgfplotslibrary{groupplots}
\usetikzlibrary{matrix,positioning}
\begin{document}
    \chapter{BJH}
 \pagebreak
\center{\huge{\hspace{2 cm}BJH}}\\
~\\
\begin{tikzpicture}
    \begin{groupplot}[group style={group size= 2 by 3,horizontal sep =1.5cm,vertical sep =1.5cm},height=6.5cm,width=8cm]

       \nextgroupplot[legend pos=north east,
        legend style={draw=none},
        y tick label style={anchor=east},       
        xticklabel=\empty,
        ytick pos=left,
        scaled ticks=true,      
        xmin=0,
        xmax=110,
        xlabel=(1),         
        ]
                \addlegendimage{empty legend};
                \addplot[red, mark=+] table{Annexes/resultats/BET/GRAPbjh1.dat};\label{plotsplot1}
                \addlegendentry{Graphite}[15 pt];               
                \coordinate (top) at (rel axis cs:0,1);% coordinate at top of the first plot            
        \nextgroupplot[legend pos=north east,
        legend style={draw=none},
        scaled ticks=false, %enlever le.10^-5 !!!!!!!!!!!!
        yticklabel pos=left,        
        ytick pos=left,
        scaled ticks=true,
        y tick label style={anchor=east},       
        xmin=0,
        xmax=140,
        xticklabel=\empty,      
xlabel=(2),     
        ]
        \addlegendimage{empty legend}
                \addplot[red, mark=+]  table{Annexes/resultats/BET/CUCEGRAPbjh1.dat};
                \addlegendentry{CuCe$_{2}$O$_{4}$/Graphite}                
        \nextgroupplot[legend pos=north east,
        legend style={draw=none},
        scaled ticks=false, %enlever le.10^-5 !!!!!!!!!!!!
        y tick label style={anchor=east},       
        ytick pos=left,
        scaled ticks=true,
        xmin=0,
        xmax=110,       
        xticklabel=\empty,  
xlabel=(3),     
        ]
                \addlegendimage{empty legend}
                \addplot[red, mark=+]  table{Annexes/resultats/BET/GRAPbjh1.dat};
                \addlegendentry{Graphite}               
        \nextgroupplot[legend pos=north east,
        legend style={draw=none},
        scaled ticks=false, %enlever le.10^-5 !!!!!!!!!!!!
        yticklabel pos=left,        
        ytick pos=left,
        scaled ticks=true,
        y tick label style={anchor=east},       
        xmin=0,
        xmax=100,       
        xticklabel=\empty,      
xlabel=(4),     
        ]
                \addlegendimage{empty legend}
                \addplot[red, mark=+]  table{Annexes/resultats/BET/CULAGRAPbjh1.dat};
                \addlegendentry{CuLa$_{2}$O$_{4}$/Graphite}               
        \nextgroupplot[legend pos=north east,
        legend style={draw=none},
        scaled ticks=false, %enlever le.10^-5 !!!!!!!!!!!!
        y tick label style={anchor=east},       
        ytick pos=left,
        scaled ticks=true,
        xmin=0,
        xmax=110,       
        xticklabel=\empty,
xlabel=(5),     
        ]
                \addlegendimage{empty legend}
                \addplot[red, mark=+]  table{Annexes/resultats/BET/GRAPbjh1.dat};
                \addlegendentry{Graphite}            
        \nextgroupplot[legend pos=north east,
        legend style={draw=none},
        scaled ticks=false, %enlever le.10^-5 !!!!!!!!!!!!
        yticklabel pos=left,
        ytick pos=left,
        scaled ticks=true,
        y tick label style={anchor=east},
        xmin=0,
        xmax=130,
        xticklabel=\empty,
xlabel=(6),     
        ]
        \addlegendimage{empty legend}
        \addplot[red, mark=+]  table{Annexes/resultats/BET/CUSMGRAPbjh1.dat};
        \addlegendentry{CuSm$_{2}$O$_{4}$/Graphite}
        \coordinate (bot) at (rel axis cs:0,0);% coordinate at bottom of the last plot
        \end{groupplot}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
   \begin{groupplot}[group style={group size= 2 by 3,horizontal sep = 1.5cm,vertical sep =1.5cm},height=6.5cm,width=8cm
]
       \nextgroupplot[legend pos=north east,
        legend style={draw=none},
        scaled ticks=false, %enlever le.10^-5 !!!!!!!!!!!!*
        %ylabeltickpos= left,       
        %yticklabel pos=left,
        y tick label style={anchor=west},               
        yticklabel pos=right,
        ytick pos=right,
        scaled ticks=true,
        every y tick scale label/.append style={xshift=1cm},
        xmin=0,
        xmax=110,       
        ]
                \addlegendimage{empty legend};
                \addplot[green, mark=o] table{Annexes/resultats/BET/GRAPbjh2.dat};\label{plotsplot2}
                \addlegendentry{Graphite}[15 pt];               
                \coordinate (top) at (rel axis cs:0,1);% coordinate at top of the first plot            
        \nextgroupplot[legend pos=north east,
        legend style={draw=none},
        scaled ticks=false, %enlever le.10^-5 !!!!!!!!!!!!
        ytick pos=right,
        y tick label style={anchor=west},       
        xmin=0,
        xmax=140,       
        yticklabel pos=right,
        scaled ticks=true,
        every y tick scale label/.append style={xshift=1cm},
        ]
                legend pos=north east,
        legend style={draw=none},
        scaled ticks=false, %enlever le.10^-5 !!!!!!!!!!!!
        \addlegendimage{empty legend}
                \addplot[green, mark=o]   table{Annexes/resultats/BET/CUCEGRAPbjh2.dat};
                \addlegendentry{CuCe$_{2}$O$_{4}$/Graphite}                
        \nextgroupplot[legend pos=north east,
        legend style={draw=none},
        scaled ticks=false, %enlever le.10^-5 !!!!!!!!!!!!
        y tick label style={anchor=west},       
        xmin=0,
        xmax=110,       
        yticklabel pos=right,       
        ytick pos=right,
        scaled ticks=true,
        every y tick scale label/.append style={xshift=1cm},
        ]
                \addlegendimage{empty legend}
                \addplot[green, mark=o]   table{Annexes/resultats/BET/GRAPbjh2.dat};
                \addlegendentry{Graphite}               
        \nextgroupplot[legend pos=north east,
        legend style={draw=none},
        scaled ticks=false, %enlever le.10^-5 !!!!!!!!!!!!
        yticklabel pos=right,       
        ytick pos=right,
        scaled ticks=true,
        y tick label style={anchor=west},       
        xmin=0,
        xmax=100,       
        every y tick scale label/.append style={xshift=1cm},
        ]
                \addlegendimage{empty legend}
                \addplot[green, mark=o]   table{Annexes/resultats/BET/CULAGRAPbjh2.dat};
                \addlegendentry{CuLa$_{2}$O$_{4}$/Graphite}               
        \nextgroupplot[xlabel={Pression Relative p/p$°$}, legend pos=north east,
        legend style={draw=none},
        scaled ticks=false, %enlever le.10^-5 !!!!!!!!!!!!
        y tick label style={anchor=west},       
        xmin=0,
        xmax=110,
        yticklabel pos=right,
        ytick pos=right,    
        scaled ticks=true,  
every y tick scale label/.append style={xshift=1cm},        
        ]
                \addlegendimage{empty legend}
                \addplot[green, mark=o]   table{Annexes/resultats/BET/GRAPbjh2.dat};
                \addlegendentry{Graphite}            
        \nextgroupplot[xlabel={Pression Relative p/p$°$},
        legend pos=north east,
        legend style={draw=none},
        scaled ticks=false, %enlever le.10^-5 !!!!!!!!!!!!
        yticklabel pos=right,
        ytick pos=right,
        scaled ticks=true,
        y tick label style={anchor=west},
        xmin=0,
        xmax=130,
        every y tick scale label/.append style={xshift=1cm},
        ]
        \addlegendimage{empty legend}
                \addplot[green, mark=o]   table{Annexes/resultats/BET/CUSMGRAPbjh2.dat};
        \addlegendentry{CuSm$_{2}$O$_{4}$/Graphite}
                \coordinate (bot) at (rel axis cs:0,0);% coordinate at bottom of the last plot
    \end{groupplot}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    \path (top-|current bounding box.west)-- node[anchor=south,rotate=90] {Quantité Adsorbée ($cm^{3}/g$ STP)} (bot-|current bounding box.west);
% legend
\path (top|-current bounding box.north)--
      coordinate(legendpos)
      (bot|-current bounding box.north);
\end{tikzpicture}
%\ref{named}
%\restoregeometry  
%***********************************************************
\chapter{Isothermes}
\pagebreak
\center{\huge{\hspace{2 cm}Isothermes}}\\
~\\
\begin{tikzpicture}
    \begin{groupplot}[group style={group size= 2 by 3},height=6.5cm,width=8cm]
       \nextgroupplot[legend pos=north west,
        legend style={draw=none},
        scaled ticks=false, %enlever le.10^-5 !!!!!!!!!!!!*
xlabel=(a),     
        ]
                \addlegendimage{empty legend};
                \addplot[red, mark=+] table{Annexes/resultats/BET/GRAP1.dat};\label{plotsplot1}
                \addplot[magenta, mark=o] table{Annexes/resultats/BET/GRAP2.dat};\label{plotsplot2}
                \addlegendentry{Graphite}[15 pt];               
                        \coordinate (top) at (rel axis cs:0,1);% coordinate at top of the first plot            
        \nextgroupplot[legend pos=north west,
        legend style={draw=none},
        scaled ticks=false, %enlever le.10^-5 !!!!!!!!!!!!
xlabel=(b),     
        ]
                legend pos=north west,
        legend style={draw=none},
        scaled ticks=false, %enlever le.10^-5 !!!!!!!!!!!!
        \addlegendimage{empty legend}
                \addplot[red, mark=+]  table{Annexes/resultats/BET/CUCEGRAP1.dat};
                \addplot[magenta, mark=o]   table{Annexes/resultats/BET/CUCEGRAP2.dat};
                \addlegendentry{CuCe$_{2}$O$_{4}$/Graphite}                
         \nextgroupplot[legend pos=north west,
        legend style={draw=none},
        scaled ticks=false, %enlever le.10^-5 !!!!!!!!!!!!
xlabel=(c),     
        ]
                \addlegendimage{empty legend}
                \addplot[red, mark=+]  table{Annexes/resultats/BET/GRAP1.dat};
                \addplot[magenta, mark=o]   table{Annexes/resultats/BET/GRAP2.dat};
                \addlegendentry{Graphite}               
              \nextgroupplot[legend pos=north west,
        legend style={draw=none},
        scaled ticks=false, %enlever le.10^-5 !!!!!!!!!!!!
xlabel=(d),     
        ]
                \addlegendimage{empty legend}
                \addplot[red, mark=+]  table{Annexes/resultats/BET/CULAGRAP1.dat};
                \addplot[magenta, mark=o]   table{Annexes/resultats/BET/CULAGRAP2.dat};
                \addlegendentry{CuLa$_{2}$O$_{4}$/Graphite}               
                      \nextgroupplot[xlabel={Pression Relative p/p$°$}, legend pos=north west,
        legend style={draw=none},
        scaled ticks=false, %enlever le.10^-5 !!!!!!!!!!!!
xlabel=(e),     
        ]
                \addlegendimage{empty legend}
                \addplot[red, mark=+]  table{Annexes/resultats/BET/GRAP1.dat};
                \addplot[magenta, mark=o]   table{Annexes/resultats/BET/GRAP2.dat};
                \addlegendentry{Graphite}            
                       \nextgroupplot[xlabel={Pression Relative p/p$°$},
        legend pos=north west,
        legend style={draw=none},
        scaled ticks=false, %enlever le.10^-5 !!!!!!!!!!!!
xlabel=(f),
]
        \addlegendimage{empty legend}
                \addplot[red, mark=+]  table{Annexes/resultats/BET/CUSMGRAP1.dat};
                \addplot[magenta, mark=o]   table{Annexes/resultats/BET/CUSMGRAP2.dat};
                %\addplot[green] table{Annexes/resultats/BET/GRAP1.dat};
        \addlegendentry{CuSm$_{2}$O$_{4}$/Graphite}
                \coordinate (bot) at (rel axis cs:0,0);% coordinate at bottom of the last plot
    \end{groupplot}
    \path (top-|current bounding box.west)-- node[anchor=south,rotate=90] {Quantité Adsorbée ($cm^{3}/g$ STP)} (bot-|current bounding box.west);
% legend
\path (top|-current bounding box.north)--
      coordinate(legendpos)
      (bot|-current bounding box.north);
\end{tikzpicture}
%\ref{named}
\end{document}

Answer 1

You can load subcaption and caption packages and use \captionof inside a node like

\node[text width=6cm,align=center,anchor=north] at ([yshift=-5mm]my plots c1r1.south) {\captionof{subfigure}{Pressure relative (P/P)\label{subplot:one}}};

The node option textwidthis needed for\captionof` to work inside the node.

\documentclass[french,12pt,oneside,openright]{memoir}

\usepackage{pgfplots}
\usepackage{caption,subcaption}  %for tables andfigures
\usepgfplotslibrary{groupplots}
\usetikzlibrary{matrix,positioning}
\begin{document}
\chapter{Some}
\section{some}
\clearpage
\begin{figure}[!h]
\renewcommand\thesubfigure{(\alph{subfigure})}
\centering
\caption{my plots}\label{fig:plots}
\begin{tikzpicture}
    \begin{groupplot}[group style={group name=my plots,group size= 2 by 3,horizontal sep =1.5cm,vertical sep =2cm},width=6cm]
       \nextgroupplot[legend pos=north east,
        legend style={draw=none},
        y tick label style={anchor=east},
        xticklabel=\empty,
        ytick pos=left,
        scaled ticks=true,
        xmin=0,
        xmax=130,
         xtick = {20,40,...,120},
        xticklabels={20,40,...,120},
         ]
                \addlegendimage{empty legend};
                \addplot[red, mark=+] {rnd};
                \addlegendentry{Graphite}[15 pt];
        \nextgroupplot[legend pos=north east,
        legend style={draw=none},
        scaled ticks=false,
        yticklabel pos=left,
        ytick pos=left,
        scaled ticks=true,
        y tick label style={anchor=east},
        xmin=0,
        xmax=130,
         xtick = {20,40,...,120},
        xticklabels={20,40,...,120},
        ]
        \addlegendimage{empty legend}
                \addplot[red, mark=+]  {rnd};
                \addlegendentry{CuCe$_{2}$O$_{4}$/Graphite}
        \nextgroupplot[legend pos=north east,
        legend style={draw=none},
        scaled ticks=false,
        y tick label style={anchor=east},
        ytick pos=left,
        scaled ticks=true,
        xmin=0,
        xmax=110,
         xtick = {20,40,...,120},
        xticklabels={20,40,...,120},
        ]
                \addlegendimage{empty legend}
                \addplot[red, mark=+]  {rnd};
                \addlegendentry{Graphite}
        \nextgroupplot[legend pos=north east,
        legend style={draw=none},
        scaled ticks=false,
        yticklabel pos=left,
        ytick pos=left,
        scaled ticks=true,
        y tick label style={anchor=east},
        xmin=0,
        xmax=100,
         xtick = {20,40,...,120},
        xticklabels={20,40,...,120},
        ]
                \addlegendimage{empty legend}
                \addplot[red, mark=+]  {rnd};
                \addlegendentry{CuLa$_{2}$O$_{4}$/Graphite}
        \nextgroupplot[legend pos=north east,
        legend style={draw=none},
        scaled ticks=false,
        y tick label style={anchor=east},
        ytick pos=left,
        scaled ticks=true,
        xmin=0,
        xmax=110,
         xtick = {20,40,...,120},
        xticklabels={20,40,...,120},
        ]
                \addlegendimage{empty legend}
                \addplot[red, mark=+]  {rnd};
                \addlegendentry{Graphite}
        \nextgroupplot[legend pos=north east,
        legend style={draw=none},
        scaled ticks=false,
        yticklabel pos=left,
        ytick pos=left,
        scaled ticks=true,
        y tick label style={anchor=east},
        xmin=0,
        xmax=130,
        xtick = {20,40,...,120},
        xticklabels={20,40,...,120},
        ]
        \addlegendimage{empty legend}
        \addplot[red, mark=+] {rnd};
        \addlegendentry{CuSm$_{2}$O$_{4}$/Graphite}
        \end{groupplot}
        \node[text width=6cm,align=center,anchor=north] at ([yshift=-5mm]my plots c1r1.south) {\captionof{subfigure}{Pressure relative (P/P)\label{subplot:one}}};
\node[text width=6cm,align=center,anchor=north] at ([yshift=-5mm]my plots c2r1.south) {\captionof{subfigure}{Pressure relative (P/P)\label{subplot:two}}};
\node[text width=6cm,align=center,anchor=north] at ([yshift=-5mm]my plots c1r2.south) {\captionof{subfigure}{Pressure relative (P/P)\label{subplot:three}}};
\node[text width=6cm,align=center,anchor=north] at ([yshift=-5mm]my plots c2r2.south) {\captionof{subfigure}{Pressure relative (P/P)\label{subplot:four}}};
\node[text width=6cm,align=center,anchor=north] at ([yshift=-5mm]my plots c1r3.south) {\captionof{subfigure}{Pressure relative (P/P)\label{subplot:five}}};
\node[text width=6cm,align=center,anchor=north] at ([yshift=-5mm]my plots c2r3.south) {\captionof{subfigure}{Pressure relative (P/P)\label{subplot:six}}};
\end{tikzpicture}
\end{figure}
\ref{subplot:one}
\end{document}

Tweaking the format of sub caption labels and reference format i further is left as an exercise.