How can I display a data series as a line in an Area Chart in Excel?
Bring the data series in as an area chart series, right click on the
area series, select "Chart Type" and change the type of the series from
Area to Line (or Scatter).
The secret is that Chart Type is a separate property of each series, not
a fixed property of whole charts. So you can mix and match series almost
any way you like, with a few exceptions.
The Combination Chart type is a bit pointless once you learn how to do
combinations yourself, and I never use it.