I'm not sure if I 100% understood your question, but here it is
t = 0:0.01:2*pi; %define interval
x = cos(t);
y = sin(t);
figure;
plot(x,y); %normal circle
%%
f = 10; %define modulation frequency
A = .2; % modulation amplitude
R = A*cos(f*t); %
x = cos(t) + R;
y = sin(t) + R;
figure;
plot(x,y); %similar to your example
%%
x = cos(t).*(1 + R);
y = sin(t).*(1 + R);
figure;
plot(x,y); %modulating the radius
