I am a bit skeptical of a chart that shows velocity increasing up to 16", then abruptly dropping at 17", then increasing at 18". Maybe that powder charge got a second wind?
This seems to support the idea that the powder is all consumed within x inches, and then the bullet velocity drops as the bullet is slowed by friction, so a bullet fired from an x+" barrel will have less velocity than one fired from an x" barrel. But that is not the way things work. Even after the powder is consumed, the hot gasses it generated continue to expand and exert pressure. How long that continues depends on many factors, but it does continue well after the powder is consumed.