Only one xlabel for matrix of plots in pgfplots

I am looking for a way to make only one xlabel for a matrix of plots with pgfplots. My current plot looks like this

and would like a xlabel where the red circle is drawn.

The code for the plot is quite long, but here goes.

\begin{figure}[ht]
\centerfloat
% The VCG 

\subbottom[VCG with no rotation. QRS-axis angle: 30 degrees]{\label{fig:0rotationFrontalVCG}%
    $\vcenter{\hbox{
    \begin{tikzpicture}
    \begin{axis}[
    title = {Frontal VCG},
    axis equal image,
    xlabel= {Right to Left},
    ylabel= {Inferior to Superior},
    ymin = -1000, ymax = 1000,
    xmin = -1000, xmax = 1000,
    ]

    % Plot the VCG
    \addplot[black] table[x = VCGX, y expr = \thisrow{VCGY}*(-1), select coords between index={239}{282}] {data/vcg_analysis_section/frontal_vcg_rotation/30degree.dat}
    [postaction={decorate, decoration={markings,
        mark=between positions 1/15 and 1 step 1/15 with {\arrow[black,line width=1.5pt]{>};}
      }}]
    ;

    % Add the circle
    \draw (axis cs:0,0) circle [black, radius=892.1472];

    % Add the axis through (0,0)
    \addplot[black, -] coordinates {(0, -895) (0, 895)};
    \addplot[black, -] coordinates {(-895, 0) (895, 0)};

    % Add the QRS-axis
    \addplot[red, ->, -triangle 60, line width = 1pt] coordinates {(0, 0) ({cos(-30)*895}, {sin(-30)*895})};

    \end{axis}
    \end{tikzpicture}

    }}$
}
\quad \quad
% The 6 ECG's
\subbottom[Lead I-III plus augmented limb leads shown in cabrera order.]{\label{fig:0rotationFrontalECG}%
$\vcenter{\hbox{
\begin{tikzpicture}

    \pgfplotsset{small}
    \matrix {

        \begin{axis}[title = Lead aVL, width=0.4\textwidth]
            \addplot[black] table[x expr=\coordindex, y=aVL] {data/vcg_analysis_section/frontal_vcg_rotation/30degree.dat};
        \end{axis}
        \pgfmatrixnextcell
        \begin{axis}[title = Lead II, width=0.4\textwidth]
            \addplot[black] table[x expr=\coordindex, y=II] {data/vcg_analysis_section/frontal_vcg_rotation/30degree.dat};
        \end{axis}
        \\
        \begin{axis}[title = Lead I, width=0.4\textwidth,ylabel=Amplitude]]
            \addplot[black] table[x expr=\coordindex, y=I] {data/vcg_analysis_section/frontal_vcg_rotation/30degree.dat};
        \end{axis}
        \pgfmatrixnextcell
        \begin{axis}[title = Lead aVF, width=0.4\textwidth]
            \addplot[black] table[x expr=\coordindex, y=aVF] {data/vcg_analysis_section/frontal_vcg_rotation/30degree.dat};
        \end{axis}
        \\
        \begin{axis}[title = Lead -aVR, width=0.4\textwidth]
            \addplot[black] table[x expr=\coordindex, y expr = \thisrow{aVR}*(-1)] {data/vcg_analysis_section/frontal_vcg_rotation/30degree.dat};
        \end{axis}
        \pgfmatrixnextcell
        \begin{axis}[title = Lead III, width=0.4\textwidth]
            \addplot[black] table[x expr=\coordindex, y=III] {data/vcg_analysis_section/frontal_vcg_rotation/30degree.dat};
        \end{axis}
        \\
    };
\end{tikzpicture}
}}$
}

\caption{Caption for the whole figure}
\label{fig:}
\end{figure}

The label should be placed in the middel of the two last plots.