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