Add option to show zero for each channel

[cbrnr]

Describe the problem

Currently, it is rather difficult to determine if a given data point lies below or above zero, which can be important e.g. when working with EEG (ERPs). Each channel has a tick on the left y-axis indicating zero, but this only help when the data point of interest is very close to this indicator.

Describe your solution

A continuous horizontal line (in light gray) would make that much easier. Since the zero point is not important for all kinds of signals, this should definitely be an option to toggle on and off.

Describe possible alternatives

I don't think there's any alternative.

Additional context

No response

[cbrnr]

PS: I think we'd also need to consider whether DC removal is on or off. When it's on, I think that indicator is not the "true" zero, it's the zero after removing DC. Not sure how to indicate this in the plot, but maybe a toggleable button in the toolbar for DC removal would be sufficient? That way, users would at least know if the visualized signals are the original signals or if they have been processed by removing their offsets.