Simple wrapper function that either calls Sweave
  for Rnw exercises or knit for Rmd exercises.
xweave(file, quiet = TRUE, encoding = "UTF-8", engine = NULL, envir = new.env(),
    pdf = TRUE, png = FALSE, svg = FALSE, height = 6, width = 6, resolution = 100,
    highlight = FALSE, ...)
character, ignored. The encoding is always assumed to be UTF-8.
character indicating whether "Sweave" (default) or
    "knitr" should be used for rendering Rnw exercises.
argument passed to knit. By default,
    or if envir = NULL a new.env() is created for each
    xweave() call.
arguments passed to
    Sweave or opts_chunk, respectively.
    In the latter case: pdf/png/svg are mapped to dev;
    height/width are mapped to fig.height/fig.width;
    and resolution is mapped to dpi. highlight is ignored for
    Sweave.
Depending on whether file has an .Rnw or .Rmd suffix,
  either Sweave or knit is called for
  weaving the file by default. Rnw exercises can optionally also be weaved by
  knit by setting engine = "knitr".
If png = TRUE or svg = TRUE when calling Sweave, then
  the resulting includegraphics statements are supplemented with the .png
  or .svg suffix of the corresponding graphics. For svg a simple
  graphics device hook .xweave_svg_grdevice is provided on-the-fly
  for plug-in into Sweave.