No vertical grid lines parallel with z-axis

[Daniel J S. <dan...@ie...>]

I'm just curious why there are no vertical grid lines for splot.  For 
example,

set grid xtics noytics ztics
splot sin(sqrt(x**2+y**2))/sqrt(x**2+y**2)

shows no vertical grid lines aligned with the x-axis on the back right 
vertical plane.

Dan