As one "equation" what does that mean? About the best I can offer is this:
ezplot(@(x,y) (x-x0).^2 + (y-y0).^2 -r^2)
Be careful that the units are equal for the two axes, else it ill not look circular.
I could also have done it using fimplicit.
syms x y
fimplicit((x-x0).^2 + (y-y0).^2 -r^2)
So, in either case, only one equation.
If you really insist on only one "variable, then you need to use a polar coordinate transformation. Thus, you can do it in terms of polar angle theta, as:
theta = linspace(0,2*pi,100);
plot(x0 + r*cos(theta),y0 + r*sin(theta),'-')
So only one variable, theta. If you are hoping for something else, something more or less, then you need to explain carefully what the goal is here.
Are you looking for a simple way to plot a circle? Perhaps try this:
cplot = @(r,x0,y0) plot(x0 + r*cos(linspace(0,2*pi)),y0 + r*sin(linspace(0,2*pi)),'-')
Now you have a little function that will plot a circle.