Conditionally print text or show graph in beamer presentation - r

Question

I have some difficulties implementing a (beamer) presentation. Everything works fine until I include a function which checks a specific condition and accordingly returns the output (graph - print text). Without that function it works fine. So how can I either graph or print the output?
\documentclass[10pt]{beamer}
\usepackage[T1]{fontenc}
\begin{document}
\begin{frame}{test}
<<echo=FALSE, fig.height = 4>>=
dates <- seq(as.Date("2015-02-13"), as.Date("2015-02-22"), by = "days")
b <- c(1,1,1,1,2,2,3,3,3,0)
c <- c(20,30,26,20,30,40,5,10,4,0)
d <- c(11,2233,12,2,22,13,23,23,100,0)
df <- data.frame(dates,b,c,d)
plot(df)
test <- function(df) {
if(sum(tail(df[2:ncol(df)], 1)) > 0) { # check only last date
return(plot(df))
} else {
print("Have a nice day!")
}
}
test(df)
#
\end{frame}
\end{document}

Answer 1

knitr wraps output in verbatim as can be seen from the TEX that the Rnw in the question produces:
\begin{frame}{test}
\begin{knitrout}
\definecolor{shadecolor}{rgb}{0.969, 0.969, 0.969}\color{fgcolor}\begin{kframe}
\begin{verbatim}
## [1] "Have a nice day!"
\end{verbatim}
\end{kframe}
\includegraphics[width=10cm,height=8cm]{figure/unnamed-chunk-2-1}
\end{knitrout}
\end{frame}
However:
It is straightforward to use Sweave or knitr with beamer; the only thing you need to be careful of is you have to add the fragile option to the frames that contain verbatim code. [Source]
Therefore, the frame needs the fragile option:
\begin{frame}[fragile]{test}
With fragile make sure not to indent \end{frame}. (This happened to me after copying the code from the question …)