Hi

A bit off topic , not for beeing a party killer ... How big plans do you have
for this pcb program , since there are so many of this kind of programs and for a very very low price??
Making the graphic is problerly the smallest part of a pcb program.

back to topic , if I understand ur problem correct .
if u want to extend ur line :

A start point
B end point
U unit vector //this vector will allways have the length 1

dx = B.X-A.X;
dy = B.Y-A.X;

U.X = dx / sqrt(dx^2+dy^2) ;
U.Y = dy / sqrt(dx^2+dy^2) ;

now u have a unit vector u can use to what u want .

if you want to make the line 1.2 mm longer in each end.
u just add the 1.2*U to your endpoints.
B.X=B.X+U.X*1.2 ;
B.Y=B.Y +U.Y*1.2 ;
A.X=A.X - U.X*1.2;
A.Y =A.Y -U.X*1.2;

so now your line is got 2.4 mm longer.

Hope I did understand you problem correct.

Best regards
TheMelter