You must have a vector that (58 x 1), which accompanies yhat (58 x 1). Let us create vectors to use as examples:
t=double(1:40000);
y=sin(t/2000)+0.1*randn(size(t));
that=[1:689:40000]'; %time vector, 58 x 1
yhat=sin(that/2000); %desired data, 58 x 1
Then you may do
yhat=plot(t,y,'b.',that,yhat,'-rx');
legend('y','yhat');
Good luck.
