As shown in the project linked above, the built-in “draw line” function fails if the inputs for x and y are provided by parameters or variables.
It does this if I rename the variables to remove the numbers:
(Ignore the set Debug's Text blocks… I haven’t updated those)
Turns out that wasn’t the problem though. Because I renamed the function variables again to add numbers back in and it still draws a straight vertical line:
So I suspect something got corrupted related to your original function variables.
It looks like the problem occurs when the local variable or parameter has the same name as parameter used by the “draw line” function.
If you change your “x1New”-style variable names back to “x1”, the problem continues.
That’s probably true. I’ll move this to the bugs category.