[R] plot with fixed axis proportion

[Thomas Steiner]

I want to plot something (eg a circle) with a fixed ratio of the x and
y axis, or (even better) with a fixed size when I print it. Output
should then be a circle (actually it'll be someting more complicated)
with radius 5cm and not an ellipse.

I'm _sure_ this is not new, but after looking 45min for a solution, I
post here...

Thanks for help
Thomas