Simply program it so that on the first click it sets a flag:
If mouseclick()=1 and flag=0
flag = 1
x = mousex()
y = mousey()
endif
To reset it, set the variable "flag" to 0.
When the flag is equal to 1, draw the line:
If flag = 1
line x,y,mousex(),mousey()
endif
Presumably you'd want to use an object rather than a line:
make object cube 1,100
scale object 1,100,100,(mousex()-x)*(mousex()-x)+(mousey()-y)*(mousey()-y)
position object 1,(mousex()+x)/2.0,0,(mousey()+y)/2.0
point object 1,mousex(),0,mousey()
You'd be using the grid coordinates rather than mousex() and mousey() if you're working in a 3D world. Just do a simple Search/Replace.
Good luck,
Calcyman
The optomist's right, The pessimist's right.