Concerning the offset, it looks to me like you moved the offset slider slider of channel 1 to a non-zero position. You see that from the marker at the very left side of the screen, where the yellow marker is at a different position as the others. Hint: a right-click on that slider sets it to zero. The little streak could be some kind of external noise.
thanks for the paper. That makes sense. I thought about sth. like that but wasn`t sure. Couldn´t check higher frequencies (limit of my function generator).
What do think about the other picture I attached yesterday where Chn1 shows a totally different offset than Chn2-4. Moreover Chn4 shows some streaks (red circle) ?