\begin{tikzpicture}
  \begin{groupplot}[
      group style={group size=1 by 2, vertical sep=0pt},
      xmin=1.5, xmax=8, ymin=-500, ymax=700,
      ylabel={P [bar]},
      width=12cm, height=8cm,
    ]
    \nextgroupplot[xtick=\empty]
    \node at (axis cs:2,600) {A};
    \addplot[mark=none,black] table[comment chars={@,\#}, x index=0, y index=1] {\relativepath tangential-avg-pure.xvg};
    \addlegendentry{$p_{T}$}
    \addplot[mark=none,red] table[comment chars={@,\#}, x index=0, y index=2] {\relativepath tangential-avg-pure.xvg};
    \addlegendentry{$p_{rr}$}
    \nextgroupplot[xlabel={r [nm]}]
    \node at (axis cs:2,600) {B};
    \addplot[mark=none,black] table[comment chars={@,\#}, x index=0, y index=1] {\relativepath tangential-avg-100f.xvg};
    \addlegendentry{$p_{T}$}
    \addplot[mark=none,red] table[comment chars={@,\#}, x index=0, y index=2] {\relativepath tangential-avg-100f.xvg};
    \addlegendentry{$p_{rr}$}
  \end{groupplot}
\end{tikzpicture}