To be honest, I've never bothered trying to chart A's weight in the red book! The hospital paediatrician had her own chart; and the community paediatrician only weighed him because I asked.
I might download the 0-4 chart at work & print it (they're better printed on A3), continue A's weight on that. I wish it were possible get hold of the figures they use to create the graphs (e.g. an Excel spreadsheet), then I could plot just the -3 & -4SD curves plus A's own curve to see how he is doing.