[HexGirlBestGirl]

Excelgraph may or may not be bugged. But due to the way that tomes work, when dual wielding a better damaging tome than your Excelgraph, you will never cast from it, thus not using the weapon charges for the Excel Crash Benefits.

Considering there's no other case where the usage of tome scaling and auto-selection works, I'm not sure if this is a bug or an oversight.

Ambidex doesn't work and only being cripple armed with my Excelgraph main handed has allowed me to properly cast from it as I desired.

[Neus]

The game will attempt to use your best casting tool always rather than your main hand/sub hand weapon if both are applicable, namely for instances where one weapon has spelledge or you have a racial effect that can let the weapon be used as a casting tool. So, this isn't really a bug, but more of the game trying to be convenient (so you don't have to toggle ambi a bunch).

I'm not really sure how this would be changed without introducing that sort of inconvenience.