The phrase "straight line" refers to a path that is not curved or inclined. It is the shortest distance between two points and is completely linear. Full definition
Students filed quietly in straight lines from modular building to modular building. (empowerms.org)
Step 2: Now raise your hips such that your body forms an almost straight line from your shoulders to your knees by squeezing your glutes / butt muscles. (exercisegoals.com)
Unfortunately, you can't always draw this kind of straight line in marketing. (slaw.ca)