I think I have it.
"Plot chart" = XY chart
Also we need to format the Line chart to have the x-axis labels aligned with
tick marks before adding the third series
But I find that the data labels are still visible after changing third
series to Primary Axis.
Also: why would we want to mix Line and XY when the dates are consecutive?
>> Stay informed about: BUG in Excel 2007 - Plot charts w/data labels