#162 transparency of parts of 3D plot

[Vadim]

Is it possible to make part of 3D plot to be partialy transparent?

Namely I want to have zero plane (z=0) to be tranparent gray.
I true this

set pm3d depthorder
splot "data" using 1:2:(($3==0) ? 1/0 : $3) notitle w pm3d, \
"data" using 1:2:(0):(0):(0):(0):(0.5) notitle w pm3d

but it gives zero plane with opaque color corresponding to pallete.