\begin{tikzpicture}
  \begin{groupplot}[
      group style={group size=1 by 2, vertical sep=0pt},
      xmin=0, xmax=6.5,
      width=12cm, height=8cm,
    ]
    \nextgroupplot[ylabel={P[bar]}, xmax=32.5, ymin=-900,ymax=900, ytick={-800,-600,...,900}, xtick=\empty]
    \addplot[mark=none,black] table[comment chars={@,\#}, x index=0, y index=1] {\relativepath rad_pres_bin1.xvg};
    \addlegendentry{$p_{\theta\theta}$}
    \addplot[mark=none,red] table[comment chars={@,\#}, x index=0, y index=2] {\relativepath rad_pres_bin1.xvg};
    \addlegendentry{$p_{\phi\phi}$}
    \addplot[mark=none,green] table[comment chars={@,\#}, x index=0, y index=3] {\relativepath rad_pres_bin1.xvg};
    \addlegendentry{$p_{rr}$}

    \nextgroupplot[
      ymin=0, ymax=19,
      xlabel={r [nm]}, ylabel={g(r)},
      ytick={0,5,10,15}
    ]
    \addplot[mark=none,orange] table[comment chars={@,\#}, x index=0, y index=1] {\relativepath chol-bin004.xvg};
    \addlegendentry{CO}
    \addplot[mark=none,green] table[comment chars={@,\#}, x index=0, y index=1] {\relativepath popc-bin004.xvg};
    \addlegendentry{POPC}
    \addplot[mark=none,magenta] table[comment chars={@,\#}, x index=0, y index=1] {\relativepath protein-bin004.xvg};
    \addlegendentry{apoA-I}
  \end{groupplot}
\end{tikzpicture}