if TRUE, plot the raster without zoom-in or zoom-out in the
graphic device: add margins if the raster is smaller than the graphic device,
zoom-in if the raster is larger than the graphic device. This feature does
not work with an unprojected (lon/lat) raster.
add
whether to add the layer to an existing plot (TRUE) or
not (FALSE).
...
bgalpha, smooth, or other arguments passed to be passed to
plotRGB