Provide a spreadsheet with one x column and two y columns.
Create an XY chart with one data series.
Open the chart for editing and select "Data range" from the context menu, goto
tab "Data series".

Click on button "Add". The right part of the dialog is filled with empty roles,
but you see nothing in the list on the left side. You can click into the empty
area and then you get a dotted border of the empty element. This dotted border
should be always visible as long as no name is entered.

Click on the role "Name" and enter a cell range in the input field "Range for
name". Click on the next role. The chosen range for name is shown in the line 
of the "Name" role, but the entry on the left side is still empty. A chosen name
should be shown immediately in the list.

Click on the role "Y-Values" and select your range. Now the field on the left
side gets the entry "Column", although you have already chosen a name for the
new data series. You must goto the role "Name" again and choose the range again
to get the data series name in the list in the left part of the dialog.

Of cause you could use the order "Y-Values"-> "X-Values"-> "Name", but that
contradicts the normal workflow from top to bottom.