Updated: Sep 17, 2020
You can create a circle in Stata. An example is shown below:
To create the graph above, please use the following commands in a do file:
twoway function y = sqrt(1 - (x)^2), /// range(-1 1) lwidth(thick) lcolor(red) /// || function y = -sqrt(1 - (x)^2), range(-1 1) /// lwidth(thick) lcolor(red) aspect(1)
Please also check out the book Speaking Stata Graphics for more useful tips.