Transforming Pitch and Roll values in iOS

Question

I have been trying to compare the pitch and roll pattern (2 second recording) readings.

What I have done is record a Pitch and Roll Values for 10 seconds. In those 10 seconds some similar motion is applied to the device.

When I plot these on graph, the values are also same.

This only happens when providing motion to the device from same orientation/position. For example, device is lying down on the table and a motion is provided to the device. All the readings are same.

But if the device is rotate 180 deg. ccw, the readings are inverted.

Is there a way I can achieve same reading for every position? Via applying some transformation formula? I have done it for acceleration values using pitch roll and yaw. But, don't know how to achieve this for pitch and roll itself.

Basically, what I want to achieve is that the Pitch and Roll values should be independent of yaw.

Here is the plot for pitch values.. all the readings are same expect the two starting from 1.5 in the graph. These were the two times when the device was rotated 180 deg. ccw

UPDATE:

I tried to store the CMAttitude in NSUSerDefaults and then applied the multiplyByInverseOfAttitude. But still the graph plot is inverse.

CMAttitude *currentAtt = motion.attitude;

        if (firstAttitude)//firstAttitude is the stored CMAttitude
        {
            [currentAtt multiplyByInverseOfAttitude:firstAttitude];
        }

        CMQuaternion quat = currentAtt.quaternion;