[FIX] OWRadviz: legible axis labels

[robertcv]

Issue

Fixes #3744

Description of changes

• Rotate variable labels according to the angel of position
• Shorten labels to 20 characters
• Show tooltip with the full label name
• Limit number of selected variables to 20

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #3809 into master will decrease coverage by <.01%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master    #3809      +/-   ##
==========================================
- Coverage   84.77%   84.76%   -0.01%     
==========================================
  Files         374      374              
  Lines       69172    69198      +26     
==========================================
+ Hits        58640    58658      +18     
- Misses      10532    10540       +8

[BlazZupan]

In case of longer, multi-word labels, there is sufficient place to split the labels to, say, two lines, and end the label with ellipsis where there is an end of the word.

For example, we could split "Gross national income (GNI) per capita" into

Gross national
income (GNI) ...

or "Estimated gross national income per capita" into

Estimated
gross national ...

In this way the width of the labels can get short her. Currently, and in the example above, labels take much space making the graph smaller.

Display of full text of the label when mouse-over is great and it should of course stay.

[robertcv]

I add the split. Is it ok?

[BlazZupan]

This looks way better. Another tiny request for change, though, before closing this PR: if the text appears on the left side of the circle than it should be right-aligned. This was irrelevant for the one-liners before. For instance, in the example above, "Gender development" on the bottom left should be right aligned. If this change is not difficult to make, please revise the PR.

[robertcv]