If I understand correctly, the OP is unfortunately looking for a stick which defies the laws of physics... at least where the diagonals are derived from the actuation of 4 switches/sensors responsible for cardinal direction sensing. The diagonal throw of a normal joystick is essentially the hypotenuse of a right triangle possessing sides of equal length. This would mean that the diagonal actuation would occur at roughly 140% of the travel required to actuate the cardinal directions.
An analog stick which selectively maps into larger chunks of directional mapping will likely also be insufficient for exactitude due to coarse areas of transition between directions.
There may be something out there which can claim do this. But whether it's always 100% accurate would still be questionable.
An analog stick which selectively maps into larger chunks of directional mapping will likely also be insufficient for exactitude due to coarse areas of transition between directions.
There may be something out there which can claim do this. But whether it's always 100% accurate would still be questionable.