I propose a solution based on the xcoffin package. The code was added at the end of the OP's code for easier comparison.
I put the four elements: plot, table and respective captions, without further handling, in xcoffins, boxes with handles which allow easy alignment with each other. I also added an \fbox around the plot to make it easy to compare widths and heights. The color rulers show the position of the captions, centered and aligned as requested.
Slightly more extensive explanations about applying xcoffin can be found in previous questions and answers to, in some way, similar problems.
Create a box using the tcolorbox package or any other? (image)
Leave space for three logos on the title page
I need a rather complex layout for a scholarly edition and don't know where to start
\documentclass[a4paper,12pt,twoside]{article}
\usepackage{geometry}
\geometry{
inner=30mm,
top=30mm,
outer=20mm,
bottom=20mm
}
\usepackage{booktabs}
\usepackage{multirow}
\usepackage{caption}
\usepackage{pgfplots}
\usepackage[no-math]{fontspec}
\setmainfont{TeX Gyre Termes}
\usepackage{unicode-math}
\setmathfont{TeX Gyre Termes Math}
\usepackage[cal=pxtx]{mathalpha}
\setmathfont[range={\mdblkcircle,\mdblksquare,\mdblkdiamond,\blacktriangle,\blacktriangledown,\smallblacktriangleleft,\smallblacktriangleright,\bigstar,\maltese}]{STIX Two Math}
\pgfplotsset{compat=newest}
\newsavebox{\tablebox}
\newlength{\tableheight}
\newenvironment{resizedtabular}[1]
{\begin{lrbox}{\tablebox}\begin{tabular}{#1}}
{\end{tabular}\end{lrbox}%
\sbox{\tablebox}{\resizebox{\textwidth}{!}{\usebox{\tablebox}}}%
\global\tableheight=\ht\tablebox
\global\advance\tableheight\dp\tablebox
\usebox{\tablebox}}
\newsavebox{\measuredSize}
\newcommand{\resizeToWidth}[2]{%
\pgfmathsetmacro{\pgfplotswidth}{#2}%
\begin{lrbox}{\measuredSize}#1\end{lrbox}%
\pgfmathsetmacro{\pgfplotswidth}{2*\pgfplotswidth-\wd\measuredSize}%
#1%
}
\newcommand{\inputPlot}{%
\begin{tikzpicture}
\begin{axis}[xmin=90,xmax=610,ymin=0,ymax=1.2,tick pos=left,height=\tableheight,width=\pgfplotswidth,xlabel=XX,ylabel=XX]
\node[above] at (129.746, 0.10363) {$\mdblkcircle$};
\node[above] at (161.767, 0.13399) {$\mdblksquare$};
\node[above] at (194.096, 0.05136) {$\mdblkdiamond$};
\node[above] at (316.332, 0.06316) {$\blacktriangle$};
\node[above] at (329.88, 0.07794) {$\blacktriangledown$};
\node[above] at (376.988, 1) {$\smallblacktriangleleft$};
\node[above] at (429.946, 0.03925) {$\smallblacktriangleright$};
\node[above] at (468.742, 0.07811) {$\bigstar$};
\node[above] at (592.825, 0.04339) {$\maltese$};
\end{axis}
\end{tikzpicture}}
\usepackage{xcoffins}
\begin{document}
\begin{table}
\begin{minipage}[c]{0.49\textwidth}
\centering
\begin{resizedtabular}{@{}ccccc@{}}
\toprule
\multirow{2}{*}{XX} & \multicolumn{4}{c}{XX} \ \cmidrule(l){2-5}
& XX & XX & XX & XX \ \cmidrule(r){1-1}
$\mdblkcircle$ & 130 & 130 & 130 & 130 \
$\mdblksquare$ & 162 & 162 & 162 & 163 \
$\mdblkdiamond$ & 194 & 194 & 192 & 195 \
$\blacktriangle$ & 316 & 318 & 317 & 318 \
$\blacktriangledown$ & 330 & 330 & 328 & 331 \
$\smallblacktriangleleft$ & 377 & 377 & 377 & 379 \
$\smallblacktriangleright$ & 430 & 430 & 429 & 432 \
$\bigstar$ & 469 & $\dagger$ & 469 & 471 \
$\maltese$ & 593 & 591 & 591 & 595 \ \bottomrule
\end{resizedtabular}
\caption{Some Caption.}
\end{minipage}%
\hfill
\begin{minipage}[c]{0.49\textwidth}
\resizeToWidth{\inputPlot}{\textwidth}
\captionof{figure}{Some Caption.}
\end{minipage}
\end{table}
%%%%%%%%**************** xcoffin solution
\textbf{The xcoffin solution}
\NewCoffin\Framex
\NewCoffin\Tablex
\NewCoffin\CaptionTablex
\NewCoffin\Plotx
\NewCoffin\CaptionPlotx
\NewCoffin\Hrule %for degugging
\SetHorizontalCoffin\Hrule{\color{red}\rule{\textwidth}{0.2pt}}
\NewCoffin\Vrule %for degugging
\SetHorizontalCoffin\Vrule{\color{green}\rule{0.2pt}{0.5\textheight}}
\SetHorizontalCoffin\Tablex{%
\begin{minipage}[c]{0.49\textwidth}
\begin{resizedtabular}{@{}ccccc@{}}
\toprule
\multirow{2}{*}{XX} & \multicolumn{4}{c}{XX} \ \cmidrule(l){2-5}
& XX & XX & XX & XX \ \cmidrule(r){1-1}
$\mdblkcircle$ & 130 & 130 & 130 & 130 \
$\mdblksquare$ & 162 & 162 & 162 & 163 \
$\mdblkdiamond$ & 194 & 194 & 192 & 195 \
$\blacktriangle$ & 316 & 318 & 317 & 318 \
$\blacktriangledown$ & 330 & 330 & 328 & 331 \
$\smallblacktriangleleft$ & 377 & 377 & 377 & 379 \
$\smallblacktriangleright$ & 430 & 430 & 429 & 432 \
$\bigstar$ & 469 & $\dagger$ & 469 & 471 \
$\maltese$ & 593 & 591 & 591 & 595 \ \bottomrule
\end{resizedtabular}
\end{minipage}
}
\SetVerticalCoffin\CaptionTablex{0.49\textwidth}{\captionof{table}{Some Caption.}}
\SetHorizontalCoffin\Plotx{%
\fbox{\begin{minipage}[c]{0.49\textwidth}
\resizeToWidth{\inputPlot}{\textwidth}
\end{minipage}}
}
\SetVerticalCoffin\CaptionPlotx{0.49\textwidth}{\captionof{figure}{Some Caption.}}
\ResizeCoffin\Plotx{0.49\textwidth}{\CoffinTotalHeight\Tablex} % adjust plot height to table height
\JoinCoffins\Framex[hc,vc]\Tablex[l,t]
\JoinCoffins\Framex[\Tablex-hc, \Tablex-b]\CaptionTablexhc,t %down shift of 2ex
\JoinCoffins\Framex[ \Tablex-r, \Tablex-t]\Plotxl,t %right shift of 10pt
\JoinCoffins\Framex[\Plotx-hc, \Plotx-b]\CaptionPlotxhc,t %down shift of 2ex
%%------------------------------- checks
\JoinCoffins\Framex[hc,vc]\Hrule[l,t] % comment after cheking alignment
\JoinCoffins\Framex[\Tablex-l,\Tablex-b]\Hrule[l,t] % comment after cheking alignment
\JoinCoffins*\Framex[\CaptionTablex-l,\CaptionTablex-vc]\Hrule[l,t] % comment after cheking alignment
\JoinCoffins\Framex[\Plotx-hc,\Plotx-t]\Vrule[r,t] % comment after cheking alignment
\JoinCoffins\Framex[\Tablex-hc,\Tablex-t]\Vrule[r,t] % comment after cheking alignment
%%-------------------------------------
\noindent\TypesetCoffin\Framex
\end{document}
The only improvement so far is the alignment of the captions. (No bad box detected from my side, compiling the above code). But we can do better.
The natural width of the table is less than 0.49\textwidth. So, by resizing that length you are also enlarging the font size, and consequently it does not match those of the plot. With some little changes you can get a more harmonic looking output (\fbox added) :
\documentclass[a4paper,12pt,twoside]{article} %only coffin
\usepackage{geometry}
\geometry{
inner=30mm,
top=30mm,
outer=20mm,
bottom=20mm
}
\usepackage{booktabs}
\usepackage{multirow}
\usepackage{caption}
\usepackage{pgfplots}
\usepackage[no-math]{fontspec}
\setmainfont{TeX Gyre Termes}
\usepackage{unicode-math}
\setmathfont{TeX Gyre Termes Math}
\usepackage[cal=pxtx]{mathalpha}
\setmathfont[range={\mdblkcircle,\mdblksquare,\mdblkdiamond,\blacktriangle,\blacktriangledown,\smallblacktriangleleft,\smallblacktriangleright,\bigstar,\maltese}]{STIX Two Math}
\pgfplotsset{compat=newest}
\usepackage{xcoffins} %<<<< added
\usepackage{calc} %<<<< added
\usepackage{kantlipsum}%<<<< added
\begin{document}
\textbf{The xcoffin solution v2}
\NewCoffin\Framex
\NewCoffin\Tablex
\NewCoffin\CaptionTablex
\NewCoffin\Plotx
\NewCoffin\CaptionPlotx
\NewCoffin\Hrule %for degugging
\SetHorizontalCoffin\Hrule{\color{red}\rule{\textwidth}{0.2pt}}
\NewCoffin\Vrule %for degugging
\SetHorizontalCoffin\Vrule{\color{green}\rule{0.2pt}{0.5\textheight}}
\SetHorizontalCoffin\Tablex{%
\begin{tabular}{@{}ccccc@{}}
\toprule
\multirow{2}{*}{XX} & \multicolumn{4}{c}{XX} \ \cmidrule(l){2-5}
& XX & XX & XX & XX \ \cmidrule(r){1-1}
$\mdblkcircle$ & 130 & 130 & 130 & 130 \
$\mdblksquare$ & 162 & 162 & 162 & 163 \
$\mdblkdiamond$ & 194 & 194 & 192 & 195 \
$\blacktriangle$ & 316 & 318 & 317 & 318 \
$\blacktriangledown$ & 330 & 330 & 328 & 331 \
$\smallblacktriangleleft$ & 377 & 377 & 377 & 379 \
$\smallblacktriangleright$ & 430 & 430 & 429 & 432 \
$\bigstar$ & 469 & $\dagger$ & 469 & 471 \
$\maltese$ & 593 & 591 & 591 & 595 \
\bottomrule
\end{tabular}
}
\newlength{\TableH}
\setlength{\TableH}{\CoffinTotalHeight\Tablex}
\SetVerticalCoffin\CaptionTablex{\CoffinWidth\Tablex}{\captionof{table}{Some Caption.\label{tab1}}}
\SetHorizontalCoffin\Plotx{%
\fbox{%
\begin{tikzpicture}
\begin{axis}[xmin=90,xmax=610,ymin=0,ymax=1.2,tick pos=left,height=\TableH,width=0.6\textwidth,xlabel=XX,ylabel=XX]
\node[above] at (129.746, 0.10363) {$\mdblkcircle$};
\node[above] at (161.767, 0.13399) {$\mdblksquare$};
\node[above] at (194.096, 0.05136) {$\mdblkdiamond$};
\node[above] at (316.332, 0.06316) {$\blacktriangle$};
\node[above] at (329.88, 0.07794) {$\blacktriangledown$};
\node[above] at (376.988, 1) {$\smallblacktriangleleft$};
\node[above] at (429.946, 0.03925) {$\smallblacktriangleright$};
\node[above] at (468.742, 0.07811) {$\bigstar$};
\node[above] at (592.825, 0.04339) {$\maltese$};
\end{axis}
\end{tikzpicture}
}
}
\ResizeCoffin\Plotx{0.6\textwidth}{\TableH}
\SetVerticalCoffin\CaptionPlotx{\CoffinWidth\Plotx}{\captionof{figure}{Some Caption.\label{fig1}}}
\JoinCoffins\Framex[hc,vc]\Tablex[l,t]
\JoinCoffins\Framex[\Tablex-hc, \Tablex-b]\CaptionTablexhc,t %down shift of 2ex
\JoinCoffins\Framex[ \Tablex-r, \Tablex-t]\Plotxl,t %right shift of 20pt
\JoinCoffins\Framex[\Plotx-hc, \Plotx-b]\CaptionPlotxhc,t %down shift of 2ex
%%------------------------------- checks
\JoinCoffins\Framex[hc,vc]\Hrule[l,t] % comment after cheking alignment
\JoinCoffins\Framex[\Tablex-l,\Tablex-b]\Hrule[l,t] % comment after cheking alignment
\JoinCoffins*\Framex[\CaptionTablex-l,\CaptionTablex-vc]\Hrule[l,t]% comment after cheking alignment
\JoinCoffins\Framex[\Plotx-hc,\Plotx-t]\Vrule[r,t] % comment after cheking alignment
\JoinCoffins\Framex[\Tablex-hc,\Tablex-t]\Vrule[r,t] % comment after cheking alignment
%%-------------------------------------
\noindent\TypesetCoffin\Framex
\newlength{\makeVspace}
\setlength{\makeVspace}{\CoffinTotalHeight\Tablex+\CoffinTotalHeight\CaptionTablex }
\vspace{\makeVspace}
\kant[1]
As shown in Table \ref{tab1} and Figure \ref{fig1}.
\end{document}
Please note that now the xcoffin code is now much cleaner, since only the the environments tabular and tikzpicture need to be retained inside the xcoffins. The height of the plot is set to be equal to the table, like you did before.
UPDATE As requested I added some text after the picture. This is the way to do it:
\noindent\TypesetCoffin\Framex will put the assemblage in the next insertion point after the title "The xcoffin solution v2", as can be seen.
The xcoffin \Framex has zero dimension and serves as a collector of the others xcoffins
In this example I chose to use \JoinCoffin* instead of \JoinCoffin, thus the size of \Framex will remain zero during the assembly. That means that if you want to put some text or other material after the figure you must add some vertical space.
For example adding, at minimum, the height of the table plus the height of the caption.
The added code will then be (packages kantlipsum and calc also added)
\newlength{\makeVspace}
\setlength{\makeVspace}{\CoffinTotalHeight\Tablex+\CoffinTotalHeight\CaptionTablex}
\vspace{\makeVspace}
\kant[1]
\begin{minipage}[b]instead of[c]for both yourminipageenvironments. On the scaling part: Either scale to a width, or to a height. Doing both gives a distorted font. The results look good, imho, just with using[b]instead of[c]as suggested earlier. – Skillmon Nov 09 '20 at 22:11