simple IMU - tilt magnetometer
It looks like the Gyro and the Magnetometer are slightly miscalibrated. In excel, it looks like they are about 135 degrees apart. Try changing your formula to be this instead:

=DEGREES(ATAN2(U3,T3))+Dcal


Where Dcal is some calibration value. It looks like the data matches up fairly well when Dcal is about 135 degrees.