[ENH] Box Plot: Add box for missing group values

[PrimozGodec]

Issue

@lanzagar and I discussed that there should be also a box for missing values in subgropus for countinuous variables as it is done for discrete.

Description of changes

When the selected variable is continuous there is now box with missing values if they are present in the subgroup. It shows the distribution of values that are missing in the subgroups variable.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #4292 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4292      +/-   ##
==========================================
+ Coverage   86.68%   86.77%   +0.09%     
==========================================
  Files         396      396              
  Lines       71510    71549      +39     
==========================================
+ Hits        61990    62090     +100     
+ Misses       9520     9459      -61

[ajdapretnar]

I was thinking whether we could simplify the user experience by instead printing, say "missing thal" or "missing sepal length". Same for splitting by missing values. This would tell the user more clearly what those missing values are.

[PrimozGodec]

I think it is a great idea. I added the names of the variables to the missing label.

[janezd]

I have one trivial suggestion, which you may feel free to reject or ignore if no other changes are made.

I think writing missing thal in lower case would like nicer here, and even when other values are written in capitals, missing thal in lower case would not stick out.

I would also put the attribute name into (single) quotes.

[PrimozGodec]

@janezd I agree with you it looks nicer and also using a single quote more clearly shows that it is not the name of the value. It is modified.