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 <dummy-var>{,<dummy-var>}
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).