usepakage{subfig}
egin{figure*}[!htb]
centering
subfigure[Derm101 data distribution]{includegraphics[width=0.5 extwidth]{derm101_stat.png}} &
subfigure[Skin-100 distribution]{includegraphics[width=0.44 extwidth]{skin100_stat.png}} \
subfigure[Derm101 data distribution]{includegraphics[width=0.5 extwidth]{derm101_stat.png}} &
subfigure[Skin-100 distribution]{includegraphics[width=0.44 extwidth]{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*}
