Did the Potters show up in that diagram anywhere? Are the Potters a pureblood family?
The Potters are not mentioned as being related to the Blacks. I had thought that this was odd since I thought that James was a pureblood and the books said more than once that pureblood families tend to be interrelated at some point. Perhaps James is not a pureblood, or pureblood enough (maybe his parents were a witch and wizard but one or more of their parents were muggles?)