dev.capture captures the current contents of a graphics device
  as a raster (bitmap) image.
dev.capture(native = FALSE)Logical. If FALSE the result is a matrix of R
    color names, if TRUE the output is returned as a
    nativeRaster object which is more efficient for plotting, but
    not portable.
NULL if the device does not support capture, otherwise a
  matrix of color names (for native = FALSE) or a
  nativeRaster object (for native = TRUE).
Not all devices support capture of the output as raster bitmaps. Typically, only image-based devices do and even not all of them.