This is a D chord played with open 5th and 4th, fret 2 3rd, fret 3 2nd, and fret 2 1rst (strings), actually the first chord I played on my recording of Window. Every make of guitar has more or less a unique spectral signature. You can see that the wave of the actual sound wave propagating through the air from the guitar (below the spectrum/Frequency Analysis) is approximately a sine wave with about 4.56 ms (0.00456 second) between each peak. This is about 220 Hz, or approximately the note A3. This was surprising to me, since I was playing a D chord (however, if you consider harmonic content there is additional A3 available as the second harmonic of the 5th string, so perhaps this accounts for the strength of A3 energy). You can see from the frequency analysis that this sound wave actually contains many different frequencies and this is normally the case in nature unless you have a pure sine wave, in which case all of the energy would be in exactly the frequency of that wave.
The vertical height of the purple peaks in the Frequency Analysis is the relative energy at each frequency, higher vertically on the graph equals stronger/louder, however, the labeling is from 0 dB downwards. The larger a negative number, the smaller the quantity, so, for example, -20 dB is larger than -37 dB. I’m not sure which type of dB they used here, probably voltage amplitude, in which case a frequency at amplitude – 20 dB is approx. 7 times greater than another frequency amplitude at -37 dB) . I labeled the more important peaks.
You can see the open 5th string A2 note about 110 Hz at about -37 dB, the open 4th string D3 approx. 146.83 Hz at about -33 dB, and the 3rd string 2nd fret A3 note approx. 220 Hz very strong at around -20 dB (as we might expect, since the time varying wave is very close to a 220 Hz pure sine wave), and the 2nd string 3rd fret D4 note approx. 293 Hz -38 dB. The A4 440 Hz energy at around -37 dB could be primarily the second harmonic of the 2nd string 2nd fret A3 (the first harmonic is defined as the fundamental frequency of a vibrating string; the second harmonic is 2X that frequency, so 2 X A3 220 Hz = 440 Hz or an A4 note). The A4 note and all of the remaining energy on the frequency analysis are higher vibration multiples of the actual notes played (harmonics). The A4 energy at 440 Hz could include components of the fourth harmonic of the open 5th string 110 Hz, i.e., 4 X 110 Hz = 440 Hz also, but there should be less energy in higher order harmonics than lower so I would assume less of a contribution from the open 5th string fourth harmonic than the 3rd string 2nd fret A3 220 Hz second harmonic.
To actually characterize the timbre of a particular instrument, you can make successive frequency analyses at a number of times after a chord or note is played and then make a 3-d plot or wire frame of that, but I don’t have that capability at present. I note that the software I used to make the analysis here is open source, i.e., free (Audacity). I also use Audacity for producing mp3 files of wav file output from my multitrack recording software.