图未完全垂直对齐

Question

我使用包创建了这两个图pgfplots，并使用包放入页面中subfig。

这是我用来创建它的一部分代码。

  \begin{figure}[htp]
\centering
        \begin{tikzpicture}[scale=0.8]
            \begin{axis}[height=7cm,
                         x post scale=1.4,
                         xmin=0,xmax=6000,ymin=0,ymax=3500,
                         grid=both,
                         ylabel=Throughput (kbit/s),
                         /pgf/number format/.cd,
                         use comma,
                         1000 sep={},
                         legend pos = north west,
                         legend cell align=left,
                         name=main plot]
                \addplot[smooth,thick,blue!70]
                    table[x=load,y=throughput]
                    {./immagini/throughput_blue.txt};
                \addplot[smooth,thick,green!70]
                    table[x=load,y=throughput]
                    {./immagini/throughput_green.txt};
                \legend{{\footnotesize Network Coding}, {\footnotesize No Network Coding}}
            \end{axis}


            \begin{axis}[height=3.8cm,
                         x post scale=1.4,
                         xmin=0,xmax=6000,ymin=0,ymax=2,
                         grid=both,
                         xlabel= Carico offerto totale (kbit/s),ylabel= Guadagno codifica,
                         /pgf/number format/.cd,
                         use comma,
                         1000 sep={},
                         at={(main plot.below south west)},yshift=-0.1cm,
                         anchor=north west]
                \addplot[smooth,thick,red!70]
                    table[x=load,y=gain]
                    {./immagini/throughput_red.txt};
            \end{axis}
        \end{tikzpicture}
\caption{Confronto throughput per lo scenario \emph{mobile cloud}}
\label{fig:throughput_nc}
\end{figure}

问题是，如您所见，图未垂直对齐。我不明白原因。错误在哪里？