Difference between "peak at" and "peak for"

This is a question of mathematical convention. You may use either construction. There is little or no difference in meaning.

In my own scientific experience, "...peaks for X=1" suggests (but only rather weakly) that X is a variable over which we may have some control, whereas "... peaks at X=1" suggests (again, only weakly) that the peaking is a matter of observation rather than control.