set xtics ("small" 0, "medium" 3, "big" 5)
plot 'file.dat' with lines lw 2 lc rgb "black"
should work.
For more information type help xtics
:
The explicit ("" , ...) form allows arbitrary tic
positions or non-numeric tic labels. In this form, the tics do not
need to be listed in numerical order. Each tic has a position,
optionally with a label. Note that the label is a string enclosed by
quotes. It may be a constant string, such as "hello", may contain
formatting information for converting the position into its label,
such as "%3f clients", or may be empty, "". See set format
for more
information. If no string is given, the default label (numerical) is
used.
An explicit tic mark has a third parameter, the level. The default
is level 0, a major tic. Level 1 generates a minor tic. Labels are
never printed for minor tics. Major and minor tics may be
auto-generated by the program or specified explicitly by the user.
Tics with level 2 and higher must be explicitly specified by the user,
and take priority over auto-generated tics. The size of tics marks
at each level is controlled by the command set tics scale
.
Examples:
set xtics ("low" 0, "medium" 50, "high" 100)
set xtics (1,2,4,8,16,32,64,128,256,512,1024)
set ytics ("bottom" 0, "" 10, "top" 20)
set ytics ("bottom" 0, "" 10 1, "top" 20)