To combine multiple figures in one figure environment in LaTeX, e.g. to show scatter plots for different groups, the usepackage subfig is there to help. It allows to create several subfigures while also adjusting the subtitles of each of the subfigures. Simple call the package in the preamble by adding

\usepackage{subfig}


For a 2×2 array of figures, define the figure environment in the following way:

\begin{figure}[!htbp]
\begin{center}
\subfloat[Panel A]{\includegraphics[height=22ex]{PanelA_figure.pdf}}
\subfloat[Panel B]{\includegraphics[height=25ex]{PanelB_figure.pdf}}\\
\subfloat[Panel C]{\includegraphics[height=22ex]{PanelC_figure.pdf}}
\subfloat[Panel D]{\includegraphics[height=25ex]{PanelD_figure.pdf}}
\end{center}
\end{figure}

Note that the double backslash \\ after Panel B forces LaTeX to insert a linebreak so that panels C and D are printed in a second line.