Sensor fusion with accelerometer and gyroscope only
Actually, the plots for the components of gravity look good. What you miss is the fact, that your overall gravity (i.e. gravity sum) is the length of the vector, that is spanned by the three components. Hence, you should calculate

$$G=\sqrt{g_x^2+g_y^2+g_z^2},$$

where $g_x,g_y,g_z$ are the three single components measured by the acelerometer. Looking at your curves, I predict the sum will be quite flat.