Captions – Top-Aligned Subfigure with Bottom-Aligned Caption

Question

This is a follow-up to an earlier question regarding left-aligning subfigure captions.

Having achieved the left-alignment goal, I would now like the caption of the subfigure to be bottom-aligned, while having the subfigure itself be top-aligned. As an example, in the figure below, subfigures (a) and (b) would share an uppermost edge, and their captions would share a lowermost edge, and similarly for subfigures (c) and (d). This is most appropriate for the overall layout style of my document and also the content of the subfigures themselves.

\documentclass{article}
\usepackage[font=footnotesize]{subcaption}
\usepackage[draft]{graphicx}

\begin{document}
\begin{figure}[!ht]
\captionsetup[subfigure]{justification=justified,singlelinecheck=false}
\centering
\begin{subfigure}[b]{0.3\textwidth}
\includegraphics[scale=0.8]{fig/workflow-S}
\caption{workflow $S$}
\end{subfigure}
\begin{subfigure}[b]{0.5\textwidth}
\includegraphics[scale=0.8]{fig/workflow-S-run}
\caption{run of $S$}
\end{subfigure}
\vskip 8pt
\begin{subfigure}[b]{0.3\textwidth}
\includegraphics[scale=0.8]{fig/workflow-S'}
\caption{workflow $S'$}
\end{subfigure}
\begin{subfigure}[b]{0.5\textwidth}
\includegraphics[scale=0.8]{fig/workflow-S'-run}
\caption{run of $S'$}
\end{subfigure}
\caption{Here's a figure}
\end{figure}
\end{document}

I've experimented with the minipage settings for subfigure, specifically using [t] instead of [b], but it seems to have no effect. I'm unsure as to whether a position of [t] would produce the right effect anyway, or whether it would shift the captions up as well.

Why is [t] not working here, and what's the right way to produce the layout I need?

The solution should generalise to an arbitrary number of subfigures per row, subfigures which are text or tables rather than images, and should not require any hard-coded sizes.

Answer 1

Here's a flexible solution; in a figure environment you define with \xsubfigure the objects you need and then arrange them as you like. Each command has as a first argument a symbolic key to be used in \makerow; the second argument defines the objects via a key-value syntax (add \label in the body of caption, if needed).

\documentclass{article}
\usepackage[font=footnotesize]{subcaption}
\usepackage[demo]{graphicx}

\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand{\xsubfigure}{ m m }
 {% #1 is a symbolic key, #2 is a list of key-value pairs
  \roly_xsubfigure:nn { #1 } { #2 }
 }
\NewDocumentCommand{\makerow}{ m }
 {% #1 is a list of symbolic keys
  \roly_makerow:n { #1 }
 }

% define the keys
\keys_define:nn { roly/subfigures }
 {
  width .tl_set:N = \l_roly_subfig_width_tl,
  body .tl_set:N = \l_roly_subfig_body_tl,
  caption .tl_set:N = \l_roly_subfig_caption_tl,
 }

% the needed variables
\dim_new:N \l_roly_row_height_dim
\box_new:N \l_roly_body_box

% this is the inner command that stores the properties
\cs_new_protected:Npn \roly_xsubfigure:nn #1 #2
 {
  \prop_if_exist:cTF { l_roly_subfig_#1_prop }
   {
    \prop_clear:c { l_roly_subfig_#1_prop }
   }
   {
    \prop_new:c { l_roly_subfig_#1_prop }
   }
  \keys_set:nn { roly/subfigures } { #2 }
  \prop_put:cnV { l_roly_subfig_#1_prop } { width } \l_roly_subfig_width_tl
  \prop_put:cnV { l_roly_subfig_#1_prop } { body } \l_roly_subfig_body_tl
  \prop_put:cnV { l_roly_subfig_#1_prop } { caption } \l_roly_subfig_caption_tl
 }

% this is the inner command for producing a row
\cs_new_protected:Npn \roly_makerow:n #1
 {
  % get the heights of the objects on a row
  \dim_zero:N \l_roly_row_height_dim
  \clist_map_inline:nn { #1 }
   {
    \hbox_set:Nn \l_roly_body_box
     {
      \prop_item:cn { l_roly_subfig_##1_prop } { body }
     }
    \dim_compare:nT { \box_ht:N \l_roly_body_box > \l_roly_row_height_dim }
     {
      \dim_set:Nn \l_roly_row_height_dim { \box_ht:N \l_roly_body_box }
     }
   }
  % produce a line
  \clist_map_inline:nn { #1 }
   {
    % a subfigure is set here
    \begin{subfigure}[t]{ \prop_item:cn { l_roly_subfig_##1_prop } { width } }
    \raggedright
    \vspace{0pt} % for top alignment
    % the body is set in a suitably dimensioned parbox
    \parbox[t][\l_roly_row_height_dim]{\textwidth}{
      \prop_item:cn { l_roly_subfig_##1_prop } { body }
    }
    % add the caption
    \caption{ \prop_get:cn { l_roly_subfig_##1_prop } { caption } }
    \end{subfigure}
    \hspace{2em} % some space between the objects in a row
   }
   \unskip\\ % end up the row
 }
\ExplSyntaxOff
\begin{document}

\begin{figure}
\captionsetup[subfigure]{justification=justified,singlelinecheck=false}
\centering

\xsubfigure{A}{
  width=0.3\textwidth,
  body={\includegraphics[width=3cm,height=2cm]{fig/workflow-S}},
  caption={workflow $S$ and some text added to go to the next line}
}
\xsubfigure{B}{
  width=0.5\textwidth,
  body={\includegraphics[width=6cm,height=2.5cm]{fig/workflow-S-run}},
  caption={run of $S$}
}
\xsubfigure{C}{
  width=0.3\textwidth,
  body={\includegraphics[width=2cm,height=1.5cm]{fig/workflow-S'}},
  caption={workflow $S'$}
}
\xsubfigure{D}{
  width=0.5\textwidth,
  body={\includegraphics[width=4cm,height=1cm]{fig/workflow-S'-run}},
  caption={run of $S'$}
}

\makerow{A,B}

\medskip

\makerow{C,D}

\caption{Here's a figure}
\end{figure}
\end{document}

Note that if you're not satisfied with the arrangement, you can rearrange the subfigures by saying, for instance,

\makerow{A,D}

\medskip

\makerow{C,B}

In the example I set height and width for every \includegraphics, as I don't have your images, but you can use the keys you prefer.