pgfplots: trim axis breaks legend positioning

Question

I have a figure that should be centered and contain three plots. One at the top, one at the bottom-left and one at the bottom-right. I used one tikzpicture with a regular begin{axis} for the top plot and a begin{groupplot} for the bottom plots. The legend for the bottom plots should be centered above the two bottom plots / below the upper plot.

\documentclass{article}
\usepackage{pgfplots}
\usepgfplotslibrary{groupplots}
\pgfplotsset{compat=1.16}

\begin{document}
\begin{figure*}[ht!]
    \centering
    \begin{tikzpicture}%[trim axis left, trim axis right, trim axis group left, trim axis group right]
    \begin{axis}[%
        name=plota,
        height=0.35\textwidth,
        legend style={
            anchor=south,
            at={(current bounding box.north-|current axis.north)},
            legend columns=2
        },
        ybar,
        x=25mm,
        ylabel={ylabel}
    ]
        \addplot[] coordinates { (0, 1) (1,2) (2,4) (3,8) };
        \addplot[] coordinates { (0, 1) (1,3) (2,9) (3,27) };
        \addlegendentry{2}
        \addlegendentry{3}
    \end{axis}
    \node[text width=.45\linewidth,align=center,anchor=north](subfiga) at (plota.below south) {subfigure text a};
    \begin{groupplot}[group style={group size=2 by 1, group name=mygroup},
        height=.375\linewidth,
        log basis x={2},
        xlabel={Width (bytes)},
    ]
    \nextgroupplot[
        name=plotb,
        at={(plota.below south west|-subfiga.south west)},
        yshift=-8mm,
        anchor=north west,
        legend to name={legend2},
        ylabel={ylabel}
    ]
        \addlegendentry{3}
        \addlegendentry{4}
        \addplot[] coordinates { (2,2) (4,5) };
        \addplot[] coordinates { (2,5) (4,7) };
    \nextgroupplot[
        name=plotc,
        at={(plota.below south east|-subfiga.south east)},
        anchor=north east,
        yticklabel pos=upper,
        yshift=-8mm,
    ]
        \addplot[] coordinates { (2,3) (4,2) };
        \addplot[] coordinates { (2,2) (4,3) };
    \end{groupplot}
    \path (plotb.above north west)-- coordinate(legendpos) (plotc.above north east);
    \node[align=center,anchor=south](legend) at (legendpos) {\ref{legend2}};

    \node[text width=.45\linewidth,align=center,anchor=north] at
        (plotb.below south) {subfigure text b};
    \node[text width=.45\linewidth,align=center,anchor=north] at
        (plotc.below south) {subfigure text c};
    \end{tikzpicture}
    \caption{caption}
\end{figure*}
\end{document}

The figure in itself basically works (but the axis is not centered with respect to the page), as long as I don't specify any of the trim axis options. If I specify all of them, the picture is centered, but the legend is off. If I specify just trim axis left and trim axis right, all hell breaks loose (i.e. the whole picture is off-center and the legend is off):

(ignore the legend overlapping the "subfigure a" text, I will resolve that with a yshift)

Plus, I get the following error that I don't understand, regardless of the axis trim options specified.

! Package pgf Error: No shape named mygroup c1r1 is known.

See the pgf package documentation for explanation.
Type  H   for immediate help.
 ...                                              

l.54        ]

Edit: Using alias instead of name fixes this warning (thanks to user marmot). Is alias documented anywhere?

Edit: This question seems related, although I wasn't able to implement the answer for the problem at hand.

Answer 1

This produces something in which the legend is positioned where I think you want to have it. I didn't touch the trims because I do not understand what you want to do with them. What I did is:

1. Replaced name=plotb and name=plotc to alias=plotb and alias=plotc to get rid of the error message ! Package pgf Error: No shape named mygroup c1r1 is known.
2. Added midway to the legendpos coordinate to have the legend in the middle.

\documentclass{article}
\usepackage{pgfplots}
\usepgfplotslibrary{groupplots}
\pgfplotsset{compat=1.16}

\begin{document}
\begin{figure*}[ht!]
    \centering
    \begin{tikzpicture}%[trim axis left, trim axis right, trim axis group left, trim axis group right]
    \begin{axis}[%
        name=plota,
        height=0.35\textwidth,
        legend style={
            anchor=south,
            at={(current bounding box.north-|current axis.north)},
            legend columns=2
        },
        ybar,
        x=25mm,
        ylabel={ylabel}
    ]
        \addplot[] coordinates { (0, 1) (1,2) (2,4) (3,8) };
        \addlegendentry{2}
        \addplot[] coordinates { (0, 1) (1,3) (2,9) (3,27) };
        \addlegendentry{3}
    \end{axis}
    \node[text width=.45\linewidth,align=center,anchor=north](subfiga) at (plota.below south) {subfigure text a};
    \begin{groupplot}[group style={group size=2 by 1, group name=mygroup},
        height=.375\linewidth,
        log basis x={2},
        xlabel={Width (bytes)},
    ]
    \nextgroupplot[
        alias=plotb,
        at={(plota.below south west|-subfiga.south west)},
        yshift=-12mm,
        anchor=north west,
        legend to name={legend2},
        ylabel={ylabel}
    ]
        \addplot[] coordinates { (2,2) (4,5) };
        \addlegendentry{3}
        \addplot[] coordinates { (2,5) (4,7) };
        \addlegendentry{4}
    \nextgroupplot[
        alias=plotc,
        at={(plota.below south east|-subfiga.south east)},
        anchor=north east,
        yticklabel pos=upper,
        yshift=-12mm,
    ]
        \addplot[] coordinates { (2,3) (4,2) };
        \addplot[] coordinates { (2,2) (4,3) };
    \end{groupplot}
    \path (plotb.above north west)-- (plotc.above north east) 
    coordinate[midway](legendpos) ;
    \node[align=center,anchor=south](legend) at (legendpos) {\ref{legend2}};

    \node[text width=.45\linewidth,align=center,anchor=north] at
        (plotb.below south) {subfigure text b};
    \node[text width=.45\linewidth,align=center,anchor=north] at
        (plotc.below south) {subfigure text c};
    \end{tikzpicture}
    \caption{caption}
\end{figure*}
\end{document}