I found the previous answers as being incomplete. Drilled down on how to display the plot in the default RStudio "Plots" window and save the plot in "png" at the same time.
In case that someday, someone might come across the same question, here is how is being done, line by line:
R:>
R:> dev.list() # fresh start. no graphical devices available at this point
NULL
R:> dev.cur() # no current device at this point
null device
1
R:> dev.new() # open graphical devices
NULL
R:> dev.list() # list them
RStudioGD quartz_off_screen
2 3
R:> png("plot50.png") # open an offscreen device as png. New device should be number 4
R:> dev.list() # the list of all graphical devices includes the newly created device, number 4
RStudioGD quartz_off_screen quartz_off_screen
2 3 4
R:> dev.cur() # NOTE: the new created device(number 4) becomes "current" automatically,
quartz_off_screen # as soon as it has been created
4
R:> dev.set(which = 2) # switch back to device 2 used to display the plot in the default RStudio
# "Plots" window
RStudioGD
2
R:> dev.cur() # indeed, RstudioGD becomes the current device after the switch step from above
RStudioGD
2
R:> dev.list() # just a check on all available devices. device 4 still in the list after
# the switch
RStudioGD quartz_off_screen quartz_off_screen
2 3 4
R:> plot(c(1:100)) # plot an example. It will be displayed in "Plots" window of RStudio
R:> dev.list() # just a check on all the available devices
RStudioGD quartz_off_screen quartz_off_screen
2 3 4
R:> dev.copy(which = 4) # copies from current device(RStudioGD) to device 4. It automatically sets
quartz_off_screen # device 4 as current
4
R:> dev.cur() # indeed , device 4 is the current device
quartz_off_screen
4
R:> dev.off() # close device 4. IMPORTANT: AT THIS POINT the plot is saved as
RStudioGD # png("plot50.png") in the current working directory.
# Three actions takes place at this point, all at once:
# 1. closes device 4
# 2. save the plot as "plot50.png"
# 3. sets the dev.next() (which is RStudioGD) as the current device
2
R:> dev.cur() # RStudioGD becomes current as soon as device 4 has been closed down.
RStudioGD
2
R:>