equal horizontal and vertical proportions in graphics

I use several different versions of R, including RGui on Windows and rstudio on Linux.  In all cases, I use graphical commands, such as image(). 

image() displays rectangles, but I want to be able to guarantee that the heights of those rectangles will always equal the widths.  Typically, the rectangles come out with notable asymmetries.

Thus, if I need to draw a perfect circle with square pixels, I usually get a squashed ellipse with oblong pixels.

I have experimented with other commands, and I don't need to continue to use image() if some other command would be more suitable.

Thanks.

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