[FIX] Radviz: Enable projection for less than two selected variables

[VesnaT]

Issue

If selecting less than two features (in features list box) to create projection, a warning is shown, which indicates inadequate setting and therefore should not be saved.

Includes

• Code changes
• Tests
• Documentation

[janezd]

I'm not sure this is the expected behavior. When the user removes all attributes (or all except one), the widget shows a warning but keeps showing the last valid visualization. The graph should be cleared.

On the other hand, why not showing a radviz with a single attribute (data instances are spread between the center and the anchor) or without any attributes (all data instances are in the center)? Of course if this doesn't cause any other problems down the road.

I lean towards the second alternative but I'm happy with both. But if you opt for the first, I think you should clear the graph, and probably also set self.selected_vars to [].

[VesnaT]

I chose the second one to be consistent with Linear Projection.

[codecov]

Codecov Report

Merging #3444 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3444      +/-   ##
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+ Coverage   83.26%   83.28%   +0.01%     
==========================================
  Files         365      365              
  Lines       64306    64329      +23     
==========================================
+ Hits        53547    53577      +30     
+ Misses      10759    10752       -7

[janezd]

What about also removing no_features = widget.Msg("At least three numeric or categorical variables are required")? It can work with one, while with 0 it would be just empty.

[janezd]

This PR can be merged as it is, though I'd prefer if you can also remove the error message I commented above. If you don't, please add an issue.

[VesnaT]

I removed the error message and added another fix.