I don't think it matters. It just provides spacing so dots for the same pilot don't overlap each other.

It looks like a result of a countif() function. So the only way to show a count of, say "3" on a chart, need to add vertical space to accommodate each dot.

The Y axis could be day of the month, but you'd still have overlapping dots for multiple M7 on a single day. So, the Y is probably random like the poster said it was.