Returns the y-value of the given joystick, which is a value between -1 and +1 and relates to how far from the center the joystick is positioned vertically. Returns 0 if the joystick is centered vertically or if the joystick is within the deadzone, if one is defined. If the joystick's y-invert is set to false, pressing down on the joystick will return negative values while pressing up will return positive ones. If the joystick's y-invert is true, the reported values are reversed.