Currently the Velocity Chart is hardcoded to show (max) last 7 sprints. Looking at the average amount of work completed in a sprint is the main value of the Velocity Chart, so the value of the Velocity Chart increases with every sprint, because the average is getting better.
If you have Sprint of two weeks length, you are talking about only 3-4 months. If vacation or christmas time is in between, you average is getting poor.
Therefore the limitation to show only last 7 sprints is bad and should be removed.