Code:
..E...
...o..
....o.
.EEE.o
....o.
...o..
..F...
With this movement, I'd say the fighter is not moving directly, and would not be able to make a charge attack
Code:
..E...
..o...
..o...
.EEE..
..o...
..o...
..F...
This would be direct movement, and I would say a charge attack could be made, provided the fighter has a way to get across the enemy line such as an athletics check to jump over them (and would of course suffer 3 attacks of opportunity).
So I think the word "directly" still implies a straight line, though the word "nearest" still implies there will be options as to which square you may choose to move directly to.