多子图设置

使用宏包subfigure

\begin{figure}[htb!]
  \centering

  \subfigure[The daily log return of Hangseng Index ]{
  \begin{minipage}[htb!]{\linewidth}
  \centering
  \includegraphics[width=0.45\linewidth]{the daily HSI _acf_pacf.pdf}
  %\caption{fig2}
  \end{minipage}%
  }%
  
  \subfigure[The weekly log return of Hangseng Index ]{
  \begin{minipage}[htb!]{\linewidth}
  \centering
  \includegraphics[width=0.45\linewidth]{the weekly HSI_acf_pacf.pdf}
  %\caption{fig2}
  \end{minipage}
  }%

  \subfigure[monthly log return of Hangseng Index ]{
  \begin{minipage}[htb!]{\linewidth}
  \centering
  \includegraphics[width=0.45\linewidth]{the monthly HSI_acf_pacf.pdf}
  %\caption{fig2}
  \end{minipage}
  }%
  \centering
  \caption{The daily/weekly/monthly log return of Hangseng Index}
\end{figure}


\begin{figure}[ht!]
  \centering
  \includegraphics[scale = 0.4]{RV_acf_pacf.pdf}
  \caption{realized volatility \{$RV_{t}$\}}
\end{figure}