[FIX] DataProjectionWidget: attribute Selected

[AndrejaKovacic]

Issue

Fixes #4369

Includes

• Code changes
• Tests

[codecov]

Codecov Report

Merging #4393 into master will increase coverage by 0.2%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #4393     +/-   ##
=========================================
+ Coverage   86.97%   87.18%   +0.2%     
=========================================
  Files         396      401      +5     
  Lines       72143    72946    +803     
=========================================
+ Hits        62749    63600    +851     
+ Misses       9394     9346     -48

[janezd]

Looks OK to me. @VesnaT, could you take a quick look, too, just to be safe?

To my understanding, the problem is that graph.get_selection returns indices into valid_data, while group_sel is "reindexed" onto input data. But I may be wrong, so .... @VesnaT :)