On the data. Vote shares, seat shares and turnout figures are compiled estimates, not an audited dataset. The US midterm seat-change figures are the least reliable numbers here. Verify against YSK/TÜİK, the House of Commons Library, and the US House Clerk before citing.
Ratings are country-specific: the three pools never played each other, so only the difficulty-adjusted score attempts a cross-country comparison, and it does so imperfectly.