Hi N34X,
Wow! Two gold stars and a green tick. Thanks!
Pity there is no kind of function "add another series" build in... it would make more sense to me.
When in doubt, cheat 😉.
Here is a document with two Sheets. The 'Engine Room' sheet does the work. The 'Presentation' sheet is what you show to your audience.

'Actual Data for Scatter Plot' table supplies data to create the scatter plot.
'Dummy for Presentation' table does nothing. it simply repeats the data in the way you want to show it.
Now move (Cut and Paste) the scatter plot and the Dummy table to the Presentation Sheet.

The Scatter Plot still uses the 'Actual Data for Scatter Plot' table on the Engine Room sheet

I find that that the Numbers approach (several small, discrete tables, each with a purpose) has a certain charm.
Contrast this with the Excel approach which uses 'islands' of data (maybe defined as arrays or ranges) in an huge 'ocean' of a sheet (thanks for that concept, Barry!)
I think of a Numbers table as a defined range in Excel.
Happy Numbering!
Regards,
Ian.