Encoding 在TeXshop中使用Knitr引擎，但带有重音字母的pdf打印标签不正确

我想编译以下Rnw文件，其中包含正态密度函数的R图

\documentclass{amsart}
\usepackage[a4paper]{geometry}
\usepackage[italian]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}    

%~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<<setup, include=FALSE, cache=FALSE>>=
opts_chunk$set(dev = "pdf", comment = NA, fig.align='center', cache=FALSE, message=FALSE, background='white')
options(replace.assign=TRUE, width=85)
knit_hooks$set(fig=function(before, options, envir){if (before) par(mar=c(4,4,.1,.1),cex.lab=.95,cex.axis=.9,mgp=c(2,.7,0),tcl=-.3)})
@
<<prepare-data, include=FALSE>>=
@
%~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


\begin{document}
This is a plot with an italian caption ``Densità'' that means ``Density''. 
<<norm, echo = FALSE, fig.width=4, fig.height=3.5>>=
curve(dnorm(x), -4, 4, xlab = 'z', ylab = 'Densità')
 @
\end{document}

我用一个标签“Densitá”得到了正确的图（在意大利语中是密度的意思，最后一个字母是重音）

我使用的是MacOSX 10.10 Yosemite，我的R语言环境是

> sessionInfo()
R version 3.1.1 (2014-07-10)
Platform: x86_64-apple-darwin13.1.0 (64-bit)

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

loaded via a namespace (and not attached):
[1] tools_3.1.1

问题在于使用TeXShop。我使用此处建议的knitr.engine： 并编译文件label.Rnw

现在，a在绘图中得到错误的标签，即“密度”变为“密度”。 然而，我在文本中得到了正确的重音字母

有人知道为什么吗

> sessionInfo()
R version 3.1.1 (2014-07-10)
Platform: x86_64-apple-darwin13.1.0 (64-bit)

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

loaded via a namespace (and not attached):
[1] tools_3.1.1