Oscar - look closely at the y-axis for the first image (that which plots both elements). There seems to be a factor of 10^4 so the ticks that we see are not 0, -5, -10, and -15 but 0, -5 x 10^4, -10 x 10^4, and -15 x 10^4. Since the blue line is at y=-9.8, then it makes sense that it is drawn at (or very close) to zero. You should probably confirm whether the -9.8 is correct and/or the calculation of the a_d elements (perhaps they are too large?).