We can assign labels to tick values, which will be used instead of the tcik values in our diagram.
The labels can be formated with latex notation.
We can define the xticks in the following way:
xticks( [-6.28, -3.14, 3.14, 6.28],
[r'$-2\pi$', r'$-\pi$', r'$+\pi$', r'$+2\pi$'])