All the “drop-outs” are where the burner stops to recalibrate the laser power. You’ll see mention of an AOPC or WOPC (optical power calibration) feature in the product specifications. This is fairly typical for many of today’s burners.
The graph you attached shows a transfer rate (green line) with a linear progression (actually, it looks more second order to me) because it is burning in constant angular velocity (CAV) mode. The rotational speed of the disc is fixed (at about 9000 rpm in this case) and the transfer rate varies as the laser moves across the disc. The circumference of the inner tracks is shorter than that of the outer tracks, but in CAV mode it completes one full rotation in the same amount of time (1/9000 minutes, or about 0.0067 seconds). Since it covers more distance in the same about of time (on the outer tracks) it can write more data in the same amount of time (higher transfer rate). The linear data density of the optical disc is constant over the entire surface (unlike magnetic data storage, where the data density frequently varies).
Your drive is burning in a zoned constant linear velocity (Z-CLV) mode. As the laser seeks towards the outer edge of the disc (optical discs burn from the hub towards the outer edge), the drive slows down the rotational speed of the disc to keep the linear speed (of the laser, with respect to the surface of the disc) constant. When the rotational speed reaches some lower threshhold, it jumps back to the higher speed, and the transfer rate goes into a higher zone (4x, 6x, 8x, etc.)
Yes, all this is completely normal.