Knitr, cairo, and ggplot2: font family 'Pro Source' not found in PostScript font database

Trying to fix the problem with otf-fonts I have (see How do I use an otf font in a ggplot2 plot? ) I found I could use these fonts with ggplot2.

Using knitr with Rmd files is fine:

---
title: "FontTest"
author: "me"
date: "22. April 2015"
output: html_document
---

```{r, echo=FALSE}
library(ggplot2)

ggplot(mtcars, aes(x=wt, y=mpg, color=factor(gear))) + geom_point() +
  ggtitle("Fuel Efficiency of 32 Cars") +
  xlab("Weight (x1000 lb)") + ylab("Miles per Gallon") +
  theme(text=element_text(size=16, family="Arial"))
```

      

        
        
        
      

    

But when I want to use LaTeX (Rnw files), I get the error:

\documentclass{article}

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

<<>>=
Sys.setenv(LANG = "en")
library(ggplot2, quietly=TRUE)
@



<<>>=
ggplot(mtcars, aes(x=wt, y=mpg, color=factor(gear))) + geom_point() +
  ggtitle("Fuel Efficiency of 32 Cars") +
  xlab("Weight (x1000 lb)") + ylab("Miles per Gallon") +
  theme(text=element_text(size=16, family="Arial"))
@

\end{document}

      

        
        
        
      

    

Here's the error:

Writing to file test.tex
Processing code chunks with options ...
 1 : echo keep.source term verbatim (test.Rnw:6)
 2 : echo keep.source term verbatim (test.Rnw:13)
Error in grid.Call.graphics(L_text, as.graphicsAnnot(x$label), x$x, x$y,  : 
  invalid font type
Calls: <Anonymous> ... drawDetails -> drawDetails.text -> grid.Call.graphics
In addition: There were 50 or more warnings (use warnings() to see the first 50)
Execution halted

      

        
        
        
      

    

I am using Mac OS X Mavericks.

Update

I found it working using Cairo:

\documentclass{article}

\begin{document}



<<>>=
  Sys.setenv(LANG = "en")
library(ggplot2, quietly=TRUE)
library(Cairo)
@



<<dev='cairo_pdf'>>=

  ggplot(mtcars, aes(x=wt, y=mpg, color=factor(gear))) + geom_point() +
  ggtitle("Fuel Efficiency of 32 Cars") +
  xlab("Weight (x1000 lb)") + ylab("Miles per Gallon") +
  theme(text=element_text(size=16, family="Source Code Pro"))

@

\end{document}

      

        
        
        
      

    

But now I am getting a lot of warnings:

## Warning in grid.Call(LtextBounds, as.graphicsAnnot(x$label), x$x,x$y, :  font family 'Source Code Pro' not found in PostScript fontdatabase

      

        
        
        
      

    

I would like to avoid suppressing these warnings. So what do I need to do to get rid of these warnings?