In Gnuplot, you can define your own functions. Functions are defined using C
programming-style definitions, with the same operators and C math library,
functions, with the exception of the **, operator, which is the
exponentiation. For your reference, a list of
are provided here.
Here is an example of the definition of a sinc function. (The sinc function
is defined as the sine of pi times x divided by pi times x, and is used
typically in communications applications.)