By default, GNUPLOT assumes that the independent variable for
the plot command line is x. x is called the dummy variable because
it is just a notation to indicate the independent variable. The
set dummy command changes this default dummy variable name. For
example, you may find it 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>
show dummy