Purpose:
#2156

Most of the performance benefit comes from removing the exception-based code (that's my bad, it was my idea in the first place). XMin and XMax are just gravy, and they also are potentially useful in and of themselves, if someone only wants to consider a function on a narrow domain, this is probably an easier way to restrict that domain then manually modifying the function.

double? Normalised_Function_F(double z)
{
    return 0 <= z && z <= 1 ? z : null;
}
var f = plt.AddFunction(Normalised_Function_F);

f.XMin = 0.3;
f.XMax = 0.7;