I am trying to plot a phase angle vs. frequency curve numerically for steady state condition of the equation 

```
x''[t] + 2*\[Zeta]* x'[t] + x[t] == Cos[w*t]
``` 
where ``` w``` is the forcing frequency, and the initial conditions are ``` x[0] == 0, x'[0] == 0```. A similar plot is shown here. [![enter image description here][1]][1]

Any help would be highly appreciated. Thanks


  [1]: https://i.sstatic.net/dXk6R.png