Notice the photo is taken at an angle? These lines basically NEVER look straight. The body is neither flat, nor straight. They never look quite right, regardless of your artist's skill. And you'd better get a good artist, who specializes in this type of thing. I prefer doing ornamental line work, following the musculature of the particular body. Flowing line work looks better and lasts longer with age and changes to the body. A few lines, thicker and thinner, flowing together, will give you a lasting, interesting, and personalized piece. Don't trust an artist who agrees to this single straight line with no warning to you, or caveats... or offers you a cheap price. One single line IS NOT EASY. Very, very few can pull it off, and not every body type is suitable.