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：