\begin{figure}[htbp]
  \centering
  \begin{tabular}{cc}
  \includegraphics[width=0.49\columnwidth]{barrel_occupancy} &
  \includegraphics[width=0.49\columnwidth]{disk3_occupancy} \\
  (a) & (b) \\
  \end{tabular}
  \caption{Mean occupancy of (a) each layer of the barrel and (b) inner, middle and outer modules of
           endcap disk 3 as a function of the number of interactions per bunch crossing in minimum-bias
           $pp$ data. Values for less than 40 interactions per bunch crossing are from 7~\TeV\
           collisions, those for higher values from 8~\TeV\ collisions, which have an occupancy about 3\%
           higher.}
  \label{fig:barrel_occupancy}
\end{figure}