Distributions: Fix invalid selection and tooltip for some histograms

[janezd]

Issue

Fixes #7102.

When all instances in a particular bar had the same value (up to the precision shown in the label), bars were shifted for one-half width to the left for cleaner visualization.

However, the position of the bar was then used to determine the bin thresholds, hence shifting it into the previous bin.

I also strongly suspect that padding between bars also caused some instances to be skipped in other histograms, too.

Description of changes

Although we could "unoffset" the bar, a cleaner solution is to store the actual range into each bar to make it independent of visualization, that may include other offsets and paddings in the future.

Unrelated: I cleaned the code so that is consistently uses _is_last_bar method, and I also added is_first_bar (however trivial it is) to make call arguments easier to understand.

Includes

• Code changes
• Tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.77%. Comparing base (2af0b98) to head (9907bfe).
⚠️ Report is 57 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7113   +/-   ##
=======================================
  Coverage   88.76%   88.77%           
=======================================
  Files         334      334           
  Lines       73659    73665    +6     
=======================================
+ Hits        65386    65393    +7     
+ Misses       8273     8272    -1     

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