5.23.58 xtics

set [m]x[<axisnumber>]tics 
         [axis|border|inward|outward|both] 
         [auto 
          | [<minimum>,] <increment[, <maximum>] 
          | ( '<label>' <position> ... ) 
         ] 

The xtics option specifies the positions of tick marks on the $x$-axis (similarly, ytics acts on the $y$-axis). One can specify:

Two examples:

set xtics 2 1 5

will set tick marks on the $x$-axis at positions 2, 3, 4 and 5.

set x2tics ("a" 2, "b" 3)

will set tick marks on the second $x$-axis at positions 2 and 3 reading ‘a’ and ‘b’ respectively.