If a group has data for neither of the years then the group won't be listed in the crosstabs. But if it has data for at least one of the year it will be listed with value in one column and null/zero in 2nd year column based on universe design. If you want zero in place of null you write IF formula or custom format the cells.
Since the dashboard bar chart can use only two dimensions I am using you need the third column for filtering or additional information purpose. You can add another column in the Webi crosstabs. So four columns will be say country, group, current year, previous year.
Thanks
Runali