Balancing robot for dummies

Ok I have one more question just to clarify something for me. The X-axis of the accelerometer should be in the rotational axis of the wheels. But should the axis from the gyro be a perpendicular axis to the X-axis of the accelerometer like an Y-axis?

Should i use the Gyro Y-axis on the IMU? I'm cute sure I should use the Y-axis...

I see to different things below, just to clarify this

// Main module   K_bot angle    angles in Quids, 10 bit ADC -----------------------------
// 5 - angle and rate calculation       display ACC_Angle and GYRO_rate

#include <math.h>

#define   GYR_Y                 0                              // Gyro Y (IMU pin #4)
#define   ACC_Z                 1                              // Acc  Z (IMU pin #7)
#define   ACC_X                 2                              // Acc  X (IMU pin #9)

The sensors values vs position should read as follow:
Horizontal ( 0° = 0 Quid ) ACC_X: 0 ACC_Z: XX GYR_X: 0
Left side (-90° = -256 Quid) ACC_X: XX ACC_Z: 0 GYR_X: 0
Right side (+90° = +256 Quid) ACC_X:-XX ACC_Z: 0 GYR_X: 0
Reversed (180° = +512 Quid) ACC_X: 0 ACC_Z:-XX GYR_X: 0