Knitr and documentclass(exam): plots not showing

[hhy_2024]

Hi everyone, I'm typing an exam using Knitr and documentclass(exam): everything works fine, except that plots are not shown. All I get is a dark rectangle instead of the plot. I used basic plots that work in documentclass(article), taken from there: Minimal examples - Examples for Rnw, Markdown, HTML and LaTeX - Yihui Xie | 谢益辉. I provide a MWE below. ggplot does not work either. Any thoughts ? Thank you !

% \documentclass[addpoints,12pt]{exam}
\documentclass[addpoints,12pt,answers]{exam}
\usepackage[left=1.5cm,right=1.5cm,top=1.5cm,bottom=1.8cm]{geometry}
\usepackage[T1]{fontenc}
\usepackage[french]{babel}
\usepackage{lmodern}
\usepackage{graphicx}
\usepackage{textcomp}
\usepackage{amsmath, amsfonts, amssymb, amsthm}
\usepackage{multirow}
\usepackage{setspace}
\usepackage{enumitem}
\usepackage[T1]{fontenc}
\usepackage{epsfig}
\usepackage{float}
\usepackage{subfig}
%\usepackage{url}
% Allow linebreak in footnote for url's
\usepackage[hyphens]{url}

%\usepackage[demo]{graphicx}
\usepackage[export]{adjustbox}

\renewcommand\questionlabel{\textbf{\thequestion.}}
\renewcommand\partlabel{\textbf{\thepartno.}}
\renewcommand\subpartlabel{\textbf{\thesubpart.}}

\renewcommand\thepartno{\thequestion.\alph{partno}}
\renewcommand\thesubpart{\thepartno.\roman{subpart}}

\pagestyle{headandfoot}
\footer{}{Page \thepage\ / \numpages}{}

%\usepackage[hyphens,spaces,obeyspaces]{url}
%\urldef{\footurl}\url{http://www.this-is-a-very-very-very-very-very-very-very-very-very-very-very-very-very-very-very-very-very-very-very-very-very-very-very-very-long.url/}
\usepackage[colorlinks]{hyperref} % optional

\begin{document}

\begin{center}
	\fbox{\fbox{\parbox{5.5in}{\centering
				Exam}}}
\end{center}

Exam graded over \numpoints\ points with \numbonuspoints\ bonus point(s).

\begin{questions}

\section{Random questions}

\question[1] Who are you ?

\begin{solution}
Mary Poppins
\end{solution}

\question[1] Can you do plots ?

\begin{solution}

<>=
set.seed(1121)
(x=rnorm(20))
mean(x);var(x)
@

<<boring-plots, fig.width=4, fig.height=4, out.width='.4\linewidth'>>=

two plots side by side (option fig.show='hold')

par(mar=c(4,4,.1,.1),cex.lab=.95,cex.axis=.9,mgp=c(2,.7,0),tcl=-.3,las=1)
boxplot(x)
hist(x,main='')
@

\end{solution}

\end{questions}

\end{document}

[jrkrideau]

I've been playing around with this for a while and don't see anything. I suspect you are running into a package conflict, maybe some but I don't see where. Ah, wait a minute. Move

\usepackage{graphicx}

to the bottom of the preamble. This does not cure the problem but rather than black boxes you should get the box-plot in the document. It may give you a clue about the rest of the problem

I have not used base graphics in so long that I have no idea if the missing histogram is a Latex or R problem.

BTW, I think you have a couple of typos in the mwe.

<>=
set.seed(1121)
(x=rnorm(20))
mean(x);var(x)
@

Should that not be

<<>>=
set.seed(1121)
(x=rnorm(20))
mean(x);var(x)
@

and I think

<<boring-plots, fig.width=4, fig.height=4, out.width='.4\linewidth'>>=

should be

<<boring-plots, fig.width=4, fig.height=4, out.width='.4\\linewidth'>>=

Also you have

\usepackage[T1]{fontenc}

twice.

[hhy_2024]

Many thanks !! It is so nice from you to have looked into it.