Blackened window instead of plot generated with Sweave [closed]

Question

I'm trying to generate two plots side by side using Sweave and the subcaption package.

The thing is, that in the created pdf I get only blackened windows instead of plots (in the separate pdf files the plots generate just fine).

Here´s what I have in my code:

\documentclass{article}

\usepackage[demo]{graphicx}
\usepackage [section]{placeins}
\usepackage{caption}
\usepackage{subcaption}

\begin{document}
\SweaveOpts{concordance=TRUE}

\section{One}
\subsection{Density plots}

\begin{figure}[h!]
 \centering

\begin{subfigure}{.5\textwidth}
 \centering 

<<label=fig1,echo=FALSE, fig=TRUE>>=
  load("data.RData")
  plot(density(data$var1, na.rm=TRUE, to=100), main="", xlab="Var1")
@

\caption{Var1}
\label{fig:fig1}
\end{subfigure}%

\begin{subfigure}{.5\textwidth}
 \centering

<<label=fig2, echo=FALSE, fig=TRUE>>=
plot(density(data$var2), main="", xlab="Var2")
@

\caption{Var2}
\label{fig:fig1}
\end{subfigure}

\caption{Var1 and Var2}
\label{fig:var12}
\end{figure}

\end{document}

Do you know what maybe causing this problem? Any bug in my code? Sorry, if my question is too trivial, but I was trying to figure it out on my own for quite a while now and just can't find a right answer.