Feature/clean visualization

[hrntsm]

Reference Issues/PRs

What does this implement/fix? Explain your changes.

clean visualization

[codeclimate]

Code Climate has analyzed commit dead5e0 and detected 2 issues on this pull request.

Here's the issue category breakdown:

Category	Count
Complexity	2

View more on Code Climate.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 116 lines in your changes are missing coverage. Please review.

Project coverage is 40.81%. Comparing base (1d56b38) to head (dead5e0).
Report is 3 commits behind head on develop.

Files	Patch %	Lines
Optuna/Visualization/Visualization.cs	0.00%	113 Missing ⚠️
Optuna/Study/Study.cs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #255      +/-   ##
===========================================
- Coverage    47.72%   40.81%   -6.91%     
===========================================
  Files           37       38       +1     
  Lines         1756     2053     +297     
  Branches       148      164      +16     
===========================================
  Hits           838      838              
- Misses         894     1191     +297     
  Partials        24       24              

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