We need to pass the role of the data sequence in order to avoid unreliable
guess work when importing static value array.

Also, not all Excel's scatter plots have real numeric X values; some have
textural X values in which case Excel switch to generating 1, 2, 3, ... as
X values.  When importing to our chart implementation, using "categories" role
in such cases instead of "values-x" results in a more faithful chart rendering.

Change-Id: If4bc1f650bb024dcd1b1b36537f457fb38404a78