[R] how to make the units in x and y axis equal?

[Haiyong Xu]

Hi there,
I encountered a problem when plotting circles. It looks weird that the 
radii in x and y axis are different since the default lengths of units 
in x and y axes are different.
Is there any way to make the units equal?
I run the following codes and got a plot attached.

symbols(rep(0,6), rep(0,6), circles = c(5, 15, 25, 35, 45, 55), inches = 
FALSE, xlim = c(-60,60), ylim = c(-60, 60))


Thanks.
Haiyong