如何根据某些条件打开/关闭特定子部分的显示？例如，仅当 xyplotFacSpecTotVarTest.pdf 存在时，我想要显示以下子部分。
我该如何做呢？
\subsection{Checksum: Factor, Specific, and Total Variance}
Perhaps the most basic test of a regression model is if the factor and specific variances sum to equal the total variance. Here, we
present this diagnostic as an xy-plot, with the sum of factor and specific variance on the x-axis and the total variance on the y-axis. We
expect the plotted points to line up tightly along the identity line.
\clearpage
\begin{figure}[!h]
\centering
\includegraphics{\Sexpr{image_dir}xyplotFacSpecTotVarTest.pdf}
\caption{Total Variance vs. Sum of Factor and Specific Variance, All Instruments Passing Exposure Model.}
\end{figure}