Splines after Cox model

[Buyadaa Oyunchimeg]

Dear Stata users,

I am trying to draw splines after the Cox model. I would like to draw the hazard ratio as a function of X as a reference point.
In the example below, to set the variable ammonia to a given reference 3.

use http://www.stata-press.com/data/cgg/reyes, clear

Any help would be greatly appreciated.
Thank you,
Oyun