You cannot have a PSD estimate on the same time scale as the original signal. That would mean that you have a PSD estimate based on 1 sample. You must sum over some interval to produce a PSD estimate.
I tried to help you by telling you that you need to create a time vector that essentially takes the midpoints of each time interval. Remember, you time jump between intervals is really the length of your window minus the number of overlapped samples, so it's 30 samples with a sampling frequency of 1000 Hz -- 0.030 sec.
I also told you that buffer() will prepend zeros and append zeros so you need to take those into account.
Fs = 1000;
t = 0:0.001:4-0.001;
x = cos(2*pi*10*t)+randn(size(t));
winsize = 200;
numoverlap = round(0.85*winsize);
win = hamming(200);
X = buffer(x,200,numoverlap);
for nn = 1:size(X,2)
[Pxx(:,nn),F] = pwelch(X(:,nn),win,length(win)/2,length(win),Fs);
end
% create a time vector
idxbegin = find(X(:,1) == 0);
numpresteps = length(idxbegin);
idxend = find(X(:,end) == 0);
numpoststeps = length(idxend);
tbegin = -(numpresteps*dt)/2;
tend = t(end)+((numpoststeps*dt))/2;
tvec = linspace(tbegin,tend,size(Pxx,2));
surf(tvec,F,10*log10(abs(Pxx)),'EdgeColor','none');
axis xy; axis tight; colormap(jet); view(0,90);
xlabel('Time (sec)');
ylabel('Frequency (Hz)');
