1. Create a spreadsheet
2. Add some data on Sheet1 in cells A1:B3
3. Add some data on Sheet2 in cells C2:D4
4. Create a chart using both regions in the second tab (Data Source)
"Sheet1.A1:B3;Sheet2.C2:D4"
=> The chart has two unnamed series

If instead you use C2:D4 also from Sheet1, you only get two series. It is always
difficult to decide what to do with non-aligned data ranges used as "one range
to split up", but at least there shouldn't be any empty series.

Additional Dev Note: It seems like when using ranges from different sheets that
after deletion the chart model is still listener to the ranges. It must remove
as listener when being deleted.