This is something I never learned how to do in R. Yes, it's kind of handy to be able to do that, but I have no clue how to do that in R.
I did a quick search again (after so many years) and the only thing I saw is this:
asp= The aspect ratio for the plot, as speciﬁed for plot.window.
See the example in plot.window.
But anyway, it's not what I expected: some param to lock the aspect ratio, so that when you change the width, the heigth is adapted accordingly to keep the same aspect ratio.
I wonder why this is not included in the base packages of R.
If you manage to do it with R (and any of its packages), tell me the solution, that I'll do my best to include it in future versions of PluginR.