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

```
x''[t] + 2 ζ 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: 

[![sample-plot][1]][1]


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