Since your imaginary line is an equation in the format y = ax+b, you can solve it like this http://www.wikihow.com/Calculate-Slope-and-Intercepts-of-a-Line
given
10,8
and
2,2
y = ax + b
so
8 = 10a + b
-
2 = 2a + b
------------
6 = 8a
a = 6/8 = 0.75
and
8 = 10a + b
8 = 10*(0.75) + b
8 = 7.5 + b
8 - 7.5 = b
0.5 = b
so
y = 0.75*x + 0.5