Fine control of the x axis tic marks is possible with the
  set xtics command. The x-axis tic marks may be turned off with the
  set noxtics command. They may be turned on (the default state) with
  set xtics.

 Syntax:
          set xtics { {, {, }} |
                      {({""}  {, {""} }...)} }
          set noxtics
          show xtics

 The , ,  form specifies that a series of tics will
 be plotted on the x axis between the x values  and 
 with an increment of . If  is not given it is assumed to be
 infinity. The increment may be negative. For example,
           set xtics 0,.5,10
 makes tics at 0, 0.5, 1, 1.5, ..., 9.5, 10.

 The ("" , ...) form allows arbitrary tic positions or
 non-numeric tic labels. A set of tics are a set of positions, each
 with its own optional label. Note that the label is a string enclosed
 by quotes, and may be a constant string, such as "hello", or contain
 formatting information for the tic number (which is the same as the
 position), such as "%3f clients". See  set format for more
 information about this case. The label may even be empty.
 Examples:
          set xtics ("low" 0, "medium" 50, "high" 100)
          set xtics (1,2,4,8,16,32,64,128,256,512,1024)
          set xtics ("bottom" 0, "" 10, "top" 20)

 Tics will only be plotted when in range.

 The  set ytics and  set noytics commands work identically.
 See also the  set format command.