Initially, this page shows a time domain representation of a 50Hz cosine waveform (top graph). The vertical axis could be voltage or current.

The bottom graph shows an alternative representation. The horizontal axis shows the frequency of the cosine wave, whilst the vertical axis is proportional to the maximum amplitude of the wave. (For the purposes of the explanations here, we can assume a linear vertical axis directly proportional to the maximum amplitude). The red line indicates the single frequency corresponding to the waveform in the top graph.

Use the slider on the diagram to investigate how the representations change as the frequency of the cosine wave changes.