set xtics <incr> set xtics <start>, <incr>, <end>
If you specify to a negative 4#4start5#5 or 4#4incr5#5 after a numerical value (e.g., rotate by 4#4angle5#5 or offset 4#4offset5#5), the parser fails because it subtracts 4#4start5#5 or 4#4incr5#5 from that value. As a workaround, specify 0-4#4start5#5 resp. 0-4#4incr5#5 in that case.
Example:
set xtics border offset 0,0.5 -5,1,5
set xtics border offset 0,0.5 0-5,1,5
set xtics offset 0,0.5 border -5,1,5
The set grid options 'front', 'back' and 'layerdefault' affect the drawing order of the xtics, too.
Examples:
Make tics at 0, 0.5, 1, 1.5, ..., 9.5, 10.
set xtics 0,.5,10
Make tics at ..., -10, -5, 0, 5, 10, ...
set xtics 5
Make tics at 1, 100, 1e4, 1e6, 1e8.
set logscale x; set xtics 1,100,1e8