Was the graph/grid an ovulation chart?
No Did it reveal that the woman can’t’ve Got pregnant by her husband but by another man?
No Artificial insemination relevant?
NoDid Isabella previously know she wasn't the man's daughter?
No Did the man know?
No The mother?
Yes Is the man the father of the baby the mother is currently carrying?
The mother is not currently pregnant, my badDid the squares reveal that she wasn't his daughter?
Yes Were they some kind of DNA test?
No A genetic matrix type thing (eg showing how parents' eye colour relates to that of the child) which made Isabella realise she must not be her father's daughter?
Not eye color, but exactly.
****SPOILER****
Reportedly, a student in biology class got in an argument with her professor because her Punnett square showed that her blood type was impossible given her parents' blood types. She went home to discuss this with her parents, and learned that her actual birth father was her uncle, with whom the mother had an affair. Divorce proceedings began.
Well done Galfisk, Enjay, and mimsy.