Use a different equation that describes the circle to make it easier.
theta = 0:0.01:2*pi;
r = 2;
x = r*cos(theta);
y = r*sin(theta);
plot(x,y),axis equal
The problem with your approach is, plot function overwrites the data. Use
plot(x,y,x,y1)
or if you want both parts to be the same color use
plot(x,y,'b',x,y1,'b')
where 'b' stands for blue.
Alternatively, after the first call to plot function, add 'hold on' to tell Matlab the next plot function should not erase the previous content within the plot.
