Android: Magnetometer data deviates

I am trying to estimate heading from the accelerometer, gyro and magnetometer data. I have implemented A Complementary Filter from here.

What I am trying is that I am holding the phone in my hand and I walked 15 steps in a straight line and I am trying to estimate Euler angles as given in the link above. But when I plot the raw data, I observe that the magnetometer data deviates. Here are the images of raw sensor data.

My question is: how do I estimate Euler angles so that they indicate I am walking in straight line.