atom has line through middle code example

Example: a line in the middle of atom editor

Its the wrap guide taking into effect telling you that your code is 
too long than preferred length. 
Go to settings >editor > preferred Line length 
(change this to whatever number fits your screen as you like). 

Scroll below and check Soft wrap or Soft wrap at preferred length 
to make it neat.

Alternatively you can disable the wrap-guide package. Hope it helps!