matlab voltage spectrum
Do you have some convenient way to transfer that waveform into MATLAB? If it's a digital scope, it should have some sort of data I/O port. It would be tedious and somewhat inaccurate to read points with your eye.
If your scope is analog, maybe you can use your computer's sound card as a digital oscilloscope. It's limited to audio frequency range, but that may be sufficient. MATLAB can input a waveform from a sound card. Modern sound cards have 16-bit converters, which give a nice spectrum.
If that's a high-voltage signal, be careful! Attenuate is to a safe level before feeding it into your computer.
To simply the MATLAB processing, try to pick equally spaced points along the time axis that includes exactly one waveform cycle. For example, if that's a 60 Hz waveform, you could collect 50 points spaced 1/3000 seconds. Then you could feed the points directly into an FFT without having to worry about applying a weighting function. Weighting functions work fine if your data includes many waveform cycles (like from your sound card), but not if it's only one cycle (you scope snapshot).