dsyleixa:
As stated, again: I do not need absolute magnetic compass headings, just relative ones to an arbitrary start value.
Unfortunately it does not show yaw yet, and so a different lib is desired for download and install.
If you are using a MPU5060 you have the ability to calculate your relative heading.
I am not going to tell you how to do it. Your image you post is the clue.
Look at your image. Your image represents a X, Y, Z 3D coordinate system. Think of a XY graph plot on a piece of paper. Actualy, just do that now, draw a vert and horz line on a sheet of paper with the lines intersecting at a 90 degree angle. Where the lines intersect that is the relative current position. If you plot a dot on the +X+Y of 1 unit up and one unit over. Now plot another point 2 units up and 2 units over, and 3 units up and 3 units over, and 4 units up and 4 units over... You should end up with a line that from +X,+Y is at 45 degrees, which is your relative heading.
The mpu6050 gives you +/- Accelerations. Those accelerations are your direction of travel vectors that can be used to determine relative heading. It's just simple math, You can do it yourself.