Group together two and two bars in pgfplots

Question

I have a bar plot and i would like to emphasize that two and two pairs belong together. So i need some padding between Cornell Box and Conference Room, and Conference Room and Sponza. Also maybe a line between the plots could help to divide them?

\documentclass{article}
\usepackage{pgfplots}
\usepackage{pgfplotstable}
\pgfplotsset{compat=newest}
\begin{document}

\begin{center}

\begin{tikzpicture}
\pgfplotstableread{
1.00 1.9946 2.8193
1.00 1.9947 2.9286
1.00 1.9837 2.8977
1     1.9857 2.9523
1     1.9706 2.8329
1     1.9854 2.9093
}\datatable

\node [align=center, 
    text width=3cm, inner sep=0.25cm] at (1.90cm, -0.40cm) {\textsc{Cornell Box}};

\node [align=center,
    text width=4cm, inner sep=0.25cm] at (5.65cm, -0.40cm) {\textsc{Conference Room}};

\node [align=center,
    text width=4cm, inner sep=0.25cm] at (9.6cm, -0.40cm) {\textsc{Sponza}};

\begin{axis}[
   ybar,
    title style={align=center},
    title={Speedup using three GPUs on a single system\\after $200$ and $1000$ iterations },
    ticks=both,
    ytick={0, 1.00, 2.00, 3.00},
    axis x line = bottom,
    axis y line = left,
    axis line style={-|},
    nodes near coords = \rotatebox{90}{{\pgfmathprintnumber[fixed zerofill, precision=3]{\pgfplotspointmeta}}},
    nodes near coords align={vertical},
    every node near coord/.append style={font=\small, fill=white, yshift=0.5mm},
    enlarge y limits={lower, value=0.1},
    enlarge y limits={upper, value=0.22},
    ylabel=Speedup,
   xtick=data,
    ymin = 0,
    ymajorgrids,
    xticklabels={ 
        $200$, 
        $1000$,
        $200$,
        $1000$,
        $200$, 
        $1000$},
    legend style={at={(0.5, -0.30)}, anchor=north, legend columns=3},
    every axis legend/.append style={nodes={right}, inner sep = 0.2cm},
   x tick label style={align=center, yshift=-0.6cm},
    enlarge x limits=0.1,
    width=13cm,
    height=7cm,
]
\pgfplotsinvokeforeach {0,...,2}{
    \addplot table [x expr=\coordindex, y index=#1] {\datatable};
}

\legend{1 GPU\hspace*{8pt}, 2 GPUs\hspace*{8pt}, 3 GPUs} 
\end{axis}
\end{tikzpicture}
\end{center}

\end{document}

Answer 1

You can move every second bar of each series slightly to the left by using

x expr={\coordindex-mod(\coordindex,2)/4}

The vertical lines I would probably draw manually, using something like the following (see How can I add a zero line to a plot? for ways of adding lines that span the whole plot area)

\draw (axis cs:1.375,0) -- ({axis cs:1.375,0}|-{rel axis cs:0.5,1});
\draw (axis cs:3.375,0) -- ({axis cs:3.375,0}|-{rel axis cs:0.5,1});

\documentclass{article}
\usepackage{pgfplots}
\usepackage{pgfplotstable}
\pgfplotsset{compat=newest}
\begin{document}

\begin{center}

\begin{tikzpicture}
\pgfplotstableread{
1.00 1.9946 2.8193 
1.00 1.9947 2.9286
1.00 1.9837 2.8977
1     1.9857 2.9523
1     1.9706 2.8329
1     1.9854 2.9093
}\datatable

\node [align=center, 
    text width=3cm, inner sep=0.25cm] at (1.90cm, -0.40cm) {\textsc{Cornell Box}};

\node [align=center,
text width=4cm, inner sep=0.25cm] at (5.65cm, -0.40cm) {\textsc{Conference Room}};

\node [align=center,
    text width=4cm, inner sep=0.25cm] at (9.6cm, -0.40cm) {\textsc{Sponza}};

\begin{axis}[
   ybar,
    title style={align=center},
    title={Speedup using three GPUs on a single system\\after $200$ and $1000$ iterations },
    ticks=both,
    ytick={0, 1.00, 2.00, 3.00},
    axis x line = bottom,
    axis y line = left,
    axis line style={-|},
    nodes near coords = \rotatebox{90}{{\pgfmathprintnumber[fixed zerofill, precision=3]{\pgfplotspointmeta}}},
    nodes near coords align={vertical},
    every node near coord/.append style={font=\small, fill=white, yshift=0.5mm},
    enlarge y limits={lower, value=0.1},
    enlarge y limits={upper, value=0.22},
    ylabel=Speedup,
   xtick=data,
    ymin = 0,
    ymajorgrids,
    xticklabels={ 
        $200$, 
        $1000$,
        $200$,
        $1000$,
        $200$, 
        $1000$},
    legend style={at={(0.5, -0.30)}, anchor=north, legend columns=3},
    every axis legend/.append style={nodes={right}, inner sep = 0.2cm},
   x tick label style={align=center, yshift=-0.6cm},
    enlarge x limits=0.1,
    width=13cm,
    height=7cm,
]
\pgfplotsinvokeforeach {0,...,2}{
    \addplot table [x expr={\coordindex-mod(\coordindex,2)/4}, y index=#1] {\datatable};
}

\draw (axis cs:1.375,0) -- ({axis cs:1.375,0}|-{rel axis cs:0.5,1});
\draw (axis cs:3.375,0) -- ({axis cs:3.375,0}|-{rel axis cs:0.5,1});
\legend{1 GPU\hspace*{8pt}, 2 GPUs\hspace*{8pt}, 3 GPUs} 
\end{axis}
\end{tikzpicture}
\end{center}

\end{document}