Q(a,x) = 1. - igamma(a,x)
The gnuplot distribution includes source code and instructions for creating a plugin library in the directory demo/plugin. You can modify the simple example file demo_plugin.c by replacing one or more of the toy example functions with an implementation of the function you are interested in. This could include invocation of functions from an external mathematical library.
The demo/plugin directory also contains source for a plugin that implements
Q(a,x). As noted above, this plugin allows earlier versions of gnuplot to
provide the same function uigamma as the current development version.
import Q(a,x) from "uigamma_plugin" uigamma(a,x) = ((x<1 || x<a) ? 1.0-igamma(a,x) : Q(a,x))