How to draw a frequency Spectrum of a periodic composite signal?

Answer 1

As correctly implied by sanjayd_411 it is best to map information from one onto a differen domain to better visualize the phenomena.

In this case we are mapping from a time domain to frequency domain by using Fourier series. http://en.wikipedia.org/wiki/Fourier_series

The signal ones mapped into the frequency domain will lose the information when in time it occurred however it will reveal amplitudes and frequency fundamental component such signal is made of.
http://homepages.inf.ed.ac.uk/rbf/CVonline/LOCAL_COPIES/OWENS/LECT4/node2.html

To draw a signal in frequency domain you must first transform the signal using Fourier transforms and then map these results in the frequency domain. See also discreet Fourier transforms. http://homepages.inf.ed.ac.uk/rbf/CVonline/LOCAL_COPIES/OWENS/LECT4/node3.html

And finally
In discreet transforms a vector represents magnitude of a component and its position represents the frequency.

Answer 2

A periodic signal has discrete frequency components. One method of drawing frequency spectrum is to obtain the Fourier Series representation of the periodic signal. The co-efficients of the Fourier Series are in general complex numbers. The magnitude of these complex numbers at different frequencies represent the magnitude vs. frequency of the periodic signal. The phase of the complex numbers represent phase vs. frequency of the periodic signal (also known as phase spectrum).