More fine-grained check of data series labels in the unit test.

Change-Id: I6b23126816bb114407f2b75545094bc297ee7cc3
bnc#812796: Don't create data series when the series has no values.

Change-Id: I92e2d7a3fab0948aea0557cf3cb65d57d48f3f59
bnc#812796: Correctly handle static value array for OOXML charts.

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