LaTeX排版指定行列的子图

在写paper时，经常遇到LaTeX排版指定行列的子图形。以2X2子图为例，代码如下：

\usepackage{subfig}
\begin{figure*}[!htb]
\centering
\subfloat[Location of Cell Towers]{\includegraphics[width=0.47\textwidth]{figures/geo_tower.pdf}} &
\subfloat[Number of Records]{\includegraphics[width=0.47\textwidth]{figures/geo_records.pdf}} \\
\subfloat[User Population]{\includegraphics[width=0.47\textwidth]{figures/geo_population.pdf}} &
\subfloat[Data Volume]{\includegraphics[width=0.47\textwidth]{figures/geo_data_volume.pdf}} \\
\caption{Geographic Distribution of Cell Towers, Number of Records, User Population and Data Volume}
\label{fig:geo_distribution}
\vspace{0.2in}
\end{figure*}

排版效果如下：