Syntax:
set dummy <dummy-var>{,<dummy-var>} show dummy
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. It may be more convenient to call a dummy variable by a more physically meaningful or conventional name. For example, when plotting time functions:
set dummy t plot sin(t), cos(t)
Examples:
set dummy u,v set dummy ,s
The second example sets the second variable to s.
The set parametric command also changes the dummy variables (to t for plot and u,v for splot).