Graph builder bins

[#1]

In Graph Builder, when a continuous variable is used as Group X with Levels(6) (e.g. Group X(:Mean rank, Levels(6))), how are the interval boundaries determined? They appear not to be equal-width or equal-frequency bins. Is the algorithm documented anywhere?

I'm preparing a manuscript and would like to describe accurately how Group X intervals are generated.

I understand that Graph Builder automatically determines the intervals when using

Group X(:Mean rank, Levels(6))

and that custom intervals require creating a separate binned column.

My question is: what algorithm does Graph Builder use to determine the automatic interval boundaries? They do not appear to be equal-width or equal-frequency bins. Is the algorithm documented anywhere?

[#2]

Essential Graphing > Graph Builder > Graph Zones > Categorical and Continuous Variables in Group X ... 

When a continuous variable is added to a grouping zone, Graph Builder uses quantiles of the data to divide the variable into five groups.