Hallo Tom. One way is to use brush/Select Data tool:
(apologize for the bad resolution).
If you want better accuracy you can do:
x = get(myplot, 'Xdata');
y = get(myplot, 'Ydata');
idx = x>=2458356 & x<=2534675; % logical indexing of the "area" you are interested
sampleX = x(idx); % x values of the area of interest
sampleY = y(idx); % y values of the area of interest.
where myplot is the handle for your plot.
i.e.
myplot = plot(..., ...)
