If you change these two lines to this I think you will get what you are looking for.
It's the odd numbers that are messing up your grid. When you only add one point, the whole thing will shift over 1/2 of a unit. By adding 2 at a time you can avoid this.
If KeyHit(20) GridSize = GridSize + 2 If KeyHit(34) GridSize = GridSize - 2
It's the odd numbers that are messing up your grid. When you only add one point, the whole thing will shift over 1/2 of a unit. By adding 2 at a time you can avoid this.