Tomodachi and the impossible graph

[TheSource]

Key games to a Nintendo system are very closely tied to the performance of the system itself. So when Wii/DS sell 1-2m in a week, these games climb substantially. Mario Kart DS / NSMB DS / Brain Training / Nintendogs and a few others consistently, year after year, sell to 10-20% of new DS purchasers and to the existing base.