[idvusers] Data Transect on 3D grid

I have a 3D grid (model output) and I'm trying to draw a Data Transect. 
However I do not have this option under the Probes display, only if I
choose one of the 2D grids (lat&lon).
What I want is a line plot of the grid values along a line at the
surface level.
I've tried this on IDV 2.7u2 and 2.8b1.
Ilan Levy