If the Marks property AtDataOnly is set as in this example, it seems to me that selection of marks seems to consider only the first series attached to an axis.
Indeed. This is because when implementing it I was unsure how to
process this case.
There are two possibilities:
1) Prepare axis marks for each series separately.
This would be useful for if the series data are spatially separated from each other.
2) Calculate a union of value ranges for all series, then generate marks for that range. This is preferable for overlapping series.
Since I do not use AtDataOnly myself, I postponed the decision
until I get some feedback.
You are the first user of this feature, so you get to decide