\usepakage{subfig} \begin{figure*}[!htb] \centering \subfigure[Derm101 data distribution]{\includegraphics[width=0.5\textwidth]{derm101_stat.png}} & \subfigure[Skin-100 distribution]{\includegraphics[width=0.44\textwidth]{skin100_stat.png}} \\ \subfigure[Derm101 data distribution]{\includegraphics[width=0.5\textwidth]{derm101_stat.png}} & \subfigure[Skin-100 distribution]{\includegraphics[width=0.44\textwidth]{skin100_stat.png}} \\ \caption{Geographic Distribution of Cell Towers, Number of Records, User Population and Data Volume} \label{fig:data_distribution} \vspace{0.2in} \end{figure*}