Subcaption spacing inside node

Question

I need to add a subcaption at the side of each plot within a \groupplots environment. After some searching I found this topic here on Tex.SE which shows how to add a subcaption to groupplots. I changed this code a bit to display the caption at the side of the plot.

The problem is, that the node needs to have a text width. Without this parameter, TikZ will yell at you. When setting a text width, however, you have to tweak the values to match the design you are going for.

In my case, the subcaption should display some numbers. As these numbers have a different number of digits, the result looks a bit weird. The cause is, that captions are always centred inside their box, in this case, the node with the given text width.

So my first question is what I could do, to left align the entire caption. I don't want to just left align the text inside the caption which can be achieved with justification = raggedright, but left align the entire block.

If this is managed, a second issue is waiting ;)

If you look at my example, you will see, that the subcaption brings some paddings with it. These paddings do not allow me, to vertically centre the node... So the second question is how to remove the subcaption padding only on the subcaptions in this plot.

MWE

\documentclass{article}

\usepackage{caption}
\usepackage{subcaption}
\usepackage{pgfplots}

\pgfplotsset{compat=1.14}
\usepgfplotslibrary{groupplots}

\begin{document}

\begin{figure}[htb!]
    \centering
    \begin{tikzpicture}
        \begin{groupplot}[group style = {
            group size = 1 by 2,
            xlabels at = edge bottom,
            xticklabels at = edge bottom,
            vertical sep = 4pt
        },
        clip = false]

            \nextgroupplot
            \node [text width = 5em, anchor = north west, draw = red] at (rel axis cs: 1.01, 0.5)
                {\subcaption{Eggs \label{fig:a}}};
            \addplot plot coordinates {(0,0) (1,1) (2,2) (3,3)};

            \nextgroupplot
            \node [text width = 5em, anchor = north west, draw = red] at (rel axis cs: 1.01, 0.5) 
                {\subcaption{Bacon \label{fig:b}}};
            \addplot plot coordinates {(0,0) (1,1) (2,4) (3,9)};

        \end{groupplot}
    \end{tikzpicture}
    \caption{Caption of figure}
\end{figure}

Figure \ref{fig:a} and \ref{fig:b}

\end{document}

Answer 1

I have a rather simple proposal for your first problem but none for your second, also because I do not fully understand what's the problem. (Can't you just set the anchor to 170, say?) Just define a node style that measures the width of the caption and sets the text width accordingly.

\documentclass{article}

\usepackage{caption}
\usepackage{subcaption}
\usepackage{pgfplots}

\pgfplotsset{compat=1.14}
\usepgfplotslibrary{groupplots}
\tikzset{subcaption/.style args={#1 with label #2}{text
width=width("#1")+0.7cm,node contents={\subcaption{#1 \label{#2}}},anchor =170}}
\begin{document}

\begin{figure}[htb!]
    \centering
    \begin{tikzpicture}
        \begin{groupplot}[group style = {
            group size = 1 by 2,
            xlabels at = edge bottom,
            xticklabels at = edge bottom,
            vertical sep = 4pt
        },
        clip = false]

            \nextgroupplot
            \node[subcaption={Eggs with label fig:a},draw =
            red,at={(rel axis cs: 1.01, 0.5)}];
            \addplot plot coordinates {(0,0) (1,1) (2,2) (3,3)};
            \nextgroupplot
            \node[subcaption={Bacon and Ham with label fig:b}, 
            draw = red,at={(rel axis cs: 1.01, 0.5)}];
            \addplot plot coordinates {(0,0) (1,1) (2,4) (3,9)};

        \end{groupplot}
    \end{tikzpicture}
    \caption{Caption of figure}
\end{figure}

Figure \ref{fig:a} and \ref{fig:b}
\end{document}