\documentclass[border = 3mmm]{standalone}
\usepackage{pgfplots}
\usepgfplotslibrary{groupplots}
\pgfplotsset{width=10cm,compat=newest}  % <<height>>

\begin{document}

\begin{tabular}{c}
\begin{tikzpicture}
    \begin{axis}[
        height=3.4cm,
        xmin=5.5,
        xmax=19.5,
        % hide y axis,
        axis y line=none,
        axis x line=bottom,
        % set the precision of the x tick labels to be 0, which means no decimals
        % xticklabel style={/pgf/number format/precision=0},
        xtick={6,7,...,19},
        axis x line shift={4pt},
        every outer x axis line/.append style={stealth-stealth},
        title={Cafe with lunch},
        xlabel={hourly activity data}]
        \addplot [only marks, black, mark=*, mark size=2pt] coordinates{(8, 1)(9, 1)(9, 2)(9, 3)(9, 4)(10, 1)(10, 2)(10, 3)(10, 4)(10, 5)(10, 6)(10, 7)(10, 8)(11, 1)(11, 2)(12, 1)(12, 2)(12, 3)(12, 4)(12, 5)(12, 6)(13, 1)(13, 2)(13, 3)(13, 4)(13, 5)(14, 1)(14, 2)(14, 3)(14, 4)(14, 5)(14, 6)(14, 7)(14, 8)(15, 1)(15, 2)(15, 3)(16, 1)(16, 2)(17, 1)};
    \end{axis}
\end{tikzpicture}
\\
\begin{tikzpicture}
    \begin{axis}[
    height=3.15cm,
    xmin=5.5,
    xmax=19.5,
    % hide y axis,
    axis y line=none,
    axis x line=bottom,
    xtick={6,7,...,19},
    axis x line shift={4pt},
    every outer x axis line/.append style={stealth-stealth},
    mark=*,
    title={Cafe with breakfast and dinner},
    xlabel={hourly activity data}]
    \addplot [only marks] coordinates{(6, 1)(6, 2)(7, 1)(7, 2)(7, 3)(7, 4)(7, 5)(8, 1)(8, 2)(8, 3)(8, 4)(8, 5)(8, 6)(9, 1)(9, 2)(9, 3)(9, 4)(10, 1)(10, 2)(10, 3)(15, 1)(16, 1)(17, 1)(17, 2)(17, 3)(17, 4)(17, 5)(17, 6)(17, 7)(18, 1)(18, 2)(18, 3)(18, 4)(18, 5)(19, 1)(19, 2)(19, 3)(19, 4)(19, 5)(19, 6)};
    \end{axis}
\end{tikzpicture}
\\
\end{tabular}

\end{document}


