The lines in question were dive-indicators. The angled lines on the side of the aircraft were to help the pilot adjust to the proper dive angle during a dive-bombing attack by matching the angle to the horizon. The Luftwaffe envisioned the Ju 88 Schnellbomber as something of jack-of-all-trades bomber, and that included the ability to make dive-bombing attacks. In addition to the canopy gradients, the Ju 88 had dive brakes- see here as well as automatic controls that allowed the aircraft to recover from its dive.