[ENH] Projections keep colors after merging values

[janezd]

Issue

Fixes #4515.

I don't like it because if a variable has 100 values, which are merged into 15, they will not use a better palette for 15 colors, but instead have a HSL-based palette with 15 hues out of 100. This rather defeats the purpose of merging. No longer important after we implement #4581.

To do:

• Improve implementation if possible. Rethink get_column which was called twice for colors and shapes, and would now be called thrice for colors. We decided, upon @VesnaT's encouragement, that the code may not be optimal, but it is more maintainable the way it is. Calling get_column_view multiple times shouldn't affect the performance because it is linear w.r.t. the number of rows.

Description of changes

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #4577 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4577      +/-   ##
==========================================
+ Coverage   83.60%   83.62%   +0.02%     
==========================================
  Files         276      276              
  Lines       55372    55455      +83     
==========================================
+ Hits        46292    46375      +83     
  Misses       9080     9080