[ENH] Scatterplot: indicate overlap of points.

[thocevar]

Issue

Implements #3028

Description of changes

Scatterplot computes the set of indices at each coordinate for later use in compute_sizes and compute_colors. The sizes are multiplied by log_2 of the number of points. The color represents the most frequent color (or one of them if there are several).

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

Codecov Report

Merging #3177 into master will decrease coverage by <.01%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master    #3177      +/-   ##
==========================================
- Coverage   82.74%   82.74%   -0.01%     
==========================================
  Files         344      344              
  Lines       59323    59328       +5     
==========================================
  Hits        49088    49088              
- Misses      10235    10240       +5

[VesnaT]

For me the updated scatter plot looks a bit confusing, since it states 'Same size', but there appear to be points of different sizes.
I suggest adding an option to Size drop down, which enables a user to set point sizes according to the number of overlapping points, and keeping the default look as it was.

[ajdapretnar]

I agree. Adding an option to Size would make a lot of sense!