[FIX] OWLinePlot: legible bottom axis labels

[VesnaT]

Issue

Fixes #3741

Description of changes

Subclass pg.AxisItem and override tickStrings() to display feature's names.

Includes

• Code changes
• Tests
• Documentation

[ajdapretnar]

I think this is really nice. ⭐️

[lanzagar]

Just writing down a couple of observations I had:

1. Right part of right-most label is cut off (e.g. heart-disease)
2. Actual width of labels not taken into account? Sometimes too much pruning even when everything would fit (e.g. housing)
3. I don't like completely omitting labels unless really necessary, even just a single letter and a dot could be enough to indicate something should have been there. And flexible ellipsing depending on available space would be ideal.
4. When some things really do need to be omitted, it would be nice to have a tooltip or similar to be able to check one specific unlabelled variable. Currently this can be done with zooming, though a bit awkwardly.

I understand that some of these issues aren't simple to solve and don't think of them as blockers that have to be addressed.

[codecov]

Codecov Report

Merging #3768 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3768      +/-   ##
==========================================
+ Coverage   84.98%   85.04%   +0.06%     
==========================================
  Files         374      374              
  Lines       69114    69103      -11     
==========================================
+ Hits        58738    58771      +33     
+ Misses      10376    10332      -44