%set up
x = 1:0.1:10;
r = sin(x);
t = 1:91;
x = mod(t,2) == 0;
m_x = mean(r(x));
m_x2 = mean(-r(~x));
% plotting
figure
plot(t(x),r(x),'b')
hold on
plot(t(~x),-r(~x),'r')
plot(t,ones(length(t),1)*m_x)
plot(t,ones(length(t),1)*m_x2)
You can do it like this; the only operative part you need to do is:
m_x = mean(r(x));
plot(t,ones(length(t),1)*m_x)
Which in your code would like slightly different since you appear to have r as a matrix instead of a vector.
