[gradsusr] Vertical cross sections, topography and displaying levels

[Fred - GMT]

Hi,

I'm using Grads to display vertical cross sections. I'd like to display 
grey area below a defined level of pressure (determined by the parameter 
PRESsfc). How can I do that?
On the cross sections, I want display a line representing the position 
of a parcel ; for example 1,5PVU. But I have this error :
> Data Request Warning:  Varying Z dimension environment...
>   but the requested variable has no Z dimension
> Cannot contour grid - all undefined values
Indeed, the parameter does not vary with altitude. How to display it ?

Thank you !
Fred