I’m assuming your variables a and b are actual values. If not, it will obviously give an error that a and b are undefined. I think you have the visibility of your figure turned off. Also I would rename your variable from i to ii in order to avoid confusion with the built in identity of i=sqrt(-1). Try this:
close all
fig=figure;
fig.Visible='on';
for ii=1:360
x1(ii)=a; %a should be some number
y1(ii)=b; %b should be some number
end
plot(x1,y1)
