By default, GNUPLOT assumes that the independent variable for the
  plot command is x, and the independent variables for the  splot
 command are x and y. They are called the dummy variables because it
 is just a notation to indicate the independent variables.
 The  set dummy command changes these default dummy variable names.
 For example, it may be more convenient to call the dummy variable t
 when plotting time functions:

         set dummy t
         plot sin(t), cos(t)

 Syntax:
         set dummy {,}
         show dummy

 Examples:
         set dummy u,v
         set dummy ,s

 to set both dummy variables to u and v or set only the second
 variable to s.

 The  set parametric command also changes the dummy variables (to t
 for  plot and u,v for  splots).