Factorial in gnuplot

Example for versions gnuplot 4.2.2

This example consists of two files: factorial.gp and run.gp to emulate loop through use of load, reread and if. Built-in factorial operator ! is used. Since return of ! operator has type real, function gprintf is used to remove trailing zeros after decimal point and decimal point itself. 

### factorial.gp
print a, '! = ', gprintf("%.0f", a!)
a = a + 1
if (a <= 16) reread

### run.gp
#!/usr/bin/env gnuplot
a = 0
load "factorial.gp"