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).