Creating a Common Legend for Subfigures with subcaption Package in LaTeX

legendsubcaptionsubfloats

I'm working on a LaTeX template, the elsarticle class from Elsivier, where I have two subfigures created using the subcaption package. I came across this answer which provides a solution for having a common legend for subfigures, but it uses the subfigure package. Unfortunately, I've already utilized the subcaption package for previous figures in my draft.

When I attempt to integrate the provided code into my document, I encounter errors like the ones shown in this image.
enter image description here

I suspect these errors may stem from conflicts between the subfigure and subcaption packages.

My main question is: How can I modify the code to have a single common legend for both subfigures while using the subcaption package?

Here's the code snippet generating the figures:

\documentclass[preprint,3p,number]{elsarticle}

\usepackage{amssymb}
\usepackage[breaklinks=true,colorlinks,bookmarks=true]{hyperref}
\usepackage[inline]{enumitem}
\usepackage{times}
\usepackage{comment}
\usepackage{epsfig}
\usepackage{graphicx}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{algorithm}
\usepackage{algpseudocode}
\usepackage{tikz}
\usepackage{pgfplots}
\usepackage{enumitem}
\setitemize{noitemsep,topsep=0pt,parsep=0pt,partopsep=0pt}
\usepackage{subcaption}

\begin{document}

\begin{frontmatter}

%% Title, authors and addresses
\title{2 subfigures with 1 common legend}



\begin{abstract}

\end{abstract}



\begin{keyword}
% keywords here, in the form: keyword \sep keyword


\end{keyword}

\end{frontmatter}

%% main text

\begin{figure}[ht!]
\begin{subfigure}{0.48\textwidth}
\begin{tikzpicture}
\begin{axis}[
    ybar=0pt,
    enlargelimits=0.15,
    width=\columnwidth,
    ymin=90,
    ylabel={Accuracy (\%)},
    symbolic x coords={VGG,GoogLeNet,DenseNet,ResNet},
    xtick=data,
    bar width=10pt,
    legend style={ 
        at={(0.5,-0.15)},
        anchor=north,
        legend columns=-1,
    },
    axis x line*=bottom,
    axis y line*=left,
]
\addplot [blue, fill, opacity=0.7, nodes near coords,every node near coord/.append style={xshift=5pt,yshift=12pt,font=\footnotesize,rotate=90, opacity=1}]coordinates {(VGG, 93.86) (GoogLeNet, 95.22) (DenseNet, 94.85) (ResNet, 93.98)};
\addplot [green, fill, opacity=0.7, nodes near coords,every node near coord/.append style={xshift=5pt,yshift=12pt,font=\footnotesize,rotate=90, opacity=1}]coordinates {(VGG, 93.81) (GoogLeNet, 95.3) (DenseNet, 94.45) (ResNet, 93.97)};
\addplot [red, fill, opacity=0.7, nodes near coords,every node near coord/.append style={xshift=5pt,yshift=12pt,font=\footnotesize,rotate=90, opacity=1}]coordinates {(VGG, 93.86) (GoogLeNet, 95.09) (DenseNet, 94.53) (ResNet, 94.17)};
\legend{Cosine, Euclide, VBD}
\end{axis}
\end{tikzpicture}
\caption{CIFAR-10.}
\label{subfig:cifar10}
\end{subfigure}
%------------------------------------------------------------------------
\qquad
\begin{subfigure}{0.48\textwidth}
\begin{tikzpicture}
\begin{axis}[
    ybar=0pt,
    enlargelimits=0.15,
    width=\columnwidth,
    ymin=90,
    ylabel={Accuracy (\%)},
    symbolic x coords={VGG,GoogLeNet,DenseNet,ResNet},
    xtick=data,
    bar width=10pt,
    legend style={ 
        at={(0.5,-0.15)},
        anchor=north,
        legend columns=-1,
    },
    axis x line*=bottom,
    axis y line*=left,
]
\addplot [blue, fill, opacity=0.7, nodes near coords,every node near coord/.append style={xshift=5pt,yshift=12pt,font=\footnotesize,rotate=90, opacity=1}]coordinates {(VGG, 93.86) (GoogLeNet, 95.22) (DenseNet, 94.85) (ResNet, 93.98)};
\addplot [green, fill, opacity=0.7, nodes near coords,every node near coord/.append style={xshift=5pt,yshift=12pt,font=\footnotesize,rotate=90, opacity=1}]coordinates {(VGG, 93.81) (GoogLeNet, 95.3) (DenseNet, 94.45) (ResNet, 93.97)};
\addplot [red, fill, opacity=0.7, nodes near coords,every node near coord/.append style={xshift=5pt,yshift=12pt,font=\footnotesize,rotate=90, opacity=1}]coordinates {(VGG, 93.86) (GoogLeNet, 95.09) (DenseNet, 94.53) (ResNet, 94.17)};
\legend{Cosine, Euclide, VBD}
\end{axis}
\end{tikzpicture}
\caption{CIFAR-100.}
\label{subfig:cifar100}
\end{subfigure}
\caption{Ablation study on distance metrics.}
\label{fig:criteria_comparison}
\end{figure}

\end{document}

It generates this figure:
enter image description here

Here's a link for reproduction.

I'd like the legend to be at the bottom of the figure, between two subfigures. Here's my expected output using Paint:
enter image description here

I appreciate any help or suggestions. Thank you!

Best Answer

This question doesn't look as a real subfigure question, but a tikz/pgfplots question.

New answer According to pgfplots manual:

4.9.7 Legends Outside Of an Axis

you can use the:

legend to name={<name>}

option in the first axis and no legend in the second one. This option doesn't draw the legend but store its code.

This code is then executed when you use \ref{<name>}. This command can be used even outside of any tikzpicture, usually just before the main figure's \caption.

Use \centering command at the beginning of figure environmemt to get it centered, as well as the subfigures.

Old answer:

If you don't absolutly need to \refer separately CIFAR-10 and CIFAR-100 (using e.g.\ref{fig:criteria_comparison}-left or -right), here is a possible workaround :

  1. Suppress the sufigures and the subcaptions.
  2. Merge the two tikzpictures into a single one, containing two axis environments.
  3. Put the secondaxis in a scope with the suitable xshift=... option
  4. Modify the anchor of the legend (at=...) in the first axis` and suppress it in the second one.
  5. Optionally, add the suppressed subcaptions as TikZ nodes in the corresponding axis or scope.
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