FFT x-axis help

Hi , I would really appreciate help with this. On my FFT graph, how do I divide the x-axis up so that it displays the correct frequencies instead of just the default numbers. I have a 4MHz square wave for example with 62,500 points in the time domain and the same for the FFT. Thanks alot in advance

An example showing your problem would help.
But may be it just a question of scaling your X vector ?