Complete missing lines using rgl grid3d

Question

Does anyone know of a way to 1) complete the missing gridlines in the grid3d call for y, and 2) draw horizontal gridlines to close the top of the grids constructed by the grid3d calls for x and y? I've played around with various combinations of pretty calls within grid3d to no avail and am wondering if this is an rgl quirk or a misspecification on my part. Additionally, I'd like to extend the vertical axis numbering to wherever the closed grids end up.

library(rgl)
cpts <- seq(0, 2, length = 40)
spts <- seq(0, 1, length = 20)
grid <- expand.grid(s=spts, c=cpts)
UFn  <- function(s,c){c^(0.5) - exp(s) + 1}
U    <- UFn(grid$s, grid$c)
open3d()
rgl.surface(x = spts, y = matrix(U,nrow = 40, ncol = 20), z = cpts, 
            coords = c(1,3,2), specular = "black")
axes3d("x", at = pretty(spts, n = 2), color = "black")
axes3d("y", at = pretty(cpts, n = 5), color = "black")
axes3d("z--", color = "black")
grid3d("x")
grid3d("y", at = pretty(spts, n = 2))
title3d(xlab ='s', ylab = 'c', zlab = 'U', color = "black")
rgl.snapshot("3d.png")

Answer 1

I would say it is a bug. You don't get any z-lines when using grid3d("y",n=2) even though it should be the same. You can work around it by using the list specification of at, setting the x element of the list, eg:

grid3d("y", at = list(x=pretty(spts, n = 2)))