Time-domain spectroscopy
Two monochromatic lights of different frequencies can combine to show interference patterns.  At time t=0, the waves interfere constructively.  This gives a maximum in the added plot.  The waves get more out of phase.  This leads to destructive interference and a minimum in the added plot.  From one peak maximum to the next is a cycle.  So when looking at two monochromatic lights, they can be graphed as two lines in the frequency-domain (this plot shows power, P(ν), as a function of frequency).  The same two lights can be plotted as the sum of their waves in the time-domain (this plot shows power, P(t), as a function of time).