Grid in xzplane

This works in gnuplot 4.6:

set grid ztics

Have a look at this article: http://gnuplot.sourceforge.net/docs_4.2/node188.html

And for a complicated example: http://www.phyast.pitt.edu/~zov1/gnuplot/html/bargraphs.html

Sadly this is currently not supported by the grid command, a feature request should probably be posted.

Anyway, as suggested by mgilson you can manually add the grid-lines with for-loops and the set arrow command. For example adding the following two lines:

set for [x = -10:10:2] arrow from x/10.0, 0, 0 to x/10.0, 0, 100 nohead lt 0
set for [z = 0:100:10] arrow from     -1, 0, z to      1, 0,   z nohead lt 0

Results in:

You may want to rotate the plot with set view. The divide-by-ten is there because floating-point increment doesn't seem to work.

Or if you wanted the grids to be on the back of the box, do something like this:

set for [x = -10:10:2] arrow from x/10.0, 100, 0 to x/10.0, 100, 100 nohead lt 0
set for [z = 0:100:10] arrow from     -1, 100, z to      1, 100,   z nohead lt 0

set for [y = 0:100:10] arrow from     -1,   y, 0 to -1,   y, 100 nohead lt 0
set for [z = 0:100:10] arrow from     -1,   0, z to -1, 100,   z nohead lt 0

Which results in:

A style note

You can replace:

set xtics axis
set ytics axis
set ztics axis

set xzeroaxis lt 1 lw 2 lc rgb "black"
set yzeroaxis lt 1 lw 2 lc rgb "black"
set zzeroaxis lt 1 lw 2 lc rgb "black"

with the equivalent:

set tics axis
set zeroaxis lt 1 lw 2 lc rgb "black"