Your "if" is not getting executed -- IF is True iff all elements of the logical expression are true -- and you undoubtedly have data in the table that are not in the eighth hour as well as some that are.
It's not clear for sure what you do intend, but
is8=(data_useful.DateTime.Hour==8); % logical addressing for Hour==8
if any(is8) % will match if are any data at Hour==8
figure()
plot(data_useful.DateTime(is8), data_useful.Sub_metering_1(is8)) % plot the subset of data
end
While the above will (probably) work to do the one specific thing, it's generally bad practice to bury "magic" constants like the 8 above in code; use variables instead. Then you will be able to change the wanted hour to be plotted simply by changing some data, perhaps tying it to user input or an argument in a function even, without having to change the code itself.
