So, you say you have applied this transformation:
x += y * cos(angle)
x = x + y * cos(angle)
This means that if you have a line that goes from (x1, y1) to (x2, y2)...
After transformation it goes from (x1 + y1 * cos(angle), y1) to (x2 + y2 * cos(angle)).
Note: I am assuming angle is a paramater of the transformation.
There are two ...