How to reference siunitx units in axis labels for ggplot2 figures?

Question

I have a knitr Rnw file (LaTeX with embedded R) which imports the siunitx package. I declare units with \\DeclareSIUnit and I would like to reference those units in axis labels in my ggplot() figures. I have tried the master branch of latex2enc but it does not do what I need. Help?

Minimum working example:

\documentclass{article}
\usepackage{siunitx}
\DeclareSIUnit{\dunit}{\SIUnitSymbolDegree Du}
\begin{document}
    <<setup, include=TRUE>>=
    library(ggplot2)
    @

    This first plot with a changed Y label should work.

    <<new-y-label, echo=TRUE>>=
    ggplot(iris, aes(Sepal.Length, Petal.Length)) + geom_line(aes(linetype=Species)) + ylab("Petal length")
    @

    I can change the label to include basic \LaTeX\ commands using the latex2exp package, which is not formally released for R version 3.4.4.

    <<latex-y-label, echo=TRUE>>=
    # devtools::install_github('stefano-meschiari/latex2exp')
    library(latex2exp)
    ggplot(iris, aes(Sepal.Length, Petal.Length)) + geom_line(aes(linetype=Species)) + ylab(TeX("$\\alpha$"))
    @

    But how do I change the Y label to reference \si{\dunit} ?

    <<does-not-work, echo=TRUE>>=
    ggplot(iris, aes(Sepal.Length, Petal.Length)) + geom_line(aes(linetype=Species)) + ylab(TeX("$\\si{\\dunit}$"))
    @

    This does not work.  Help!
\end{document}

Answer 1

I ended up asking my question on RStudio Community as well: https://community.rstudio.com/t/how-to-reference-siunitx-units-in-axis-labels-for-ggplot2-figures/8038/8

I got two answers, both of which generated useful output, but in my opinion the best answer was the last one by baptiste. There is a non-trivial increase in compilation time, but the macros are used correctly and as a bonus the same font is used in the figure as in the rest of the document. Here is a modified version of my MWE:

\documentclass{article}
\usepackage{siunitx}
\DeclareSIUnit{\dunit}{\SIUnitSymbolDegree Du}
\begin{document}
    <<setup, include=TRUE>>=
    library(ggplot2)
    @

    <<works-and-looks-good-too, echo=TRUE, dev='tikz'>>=
    library(tikzDevice)
    ggplot(iris, aes(Sepal.Length, Petal.Length)) + geom_line(aes(linetype=Species)) + ylab("$\\si{\\dunit}$")
    @

\end{document}