how excel calculates the major units on an axis when MajorUnitIsAuto property set to true
 

I need to know, how excel calculates the major units on an axis when
MajorUnitIsAuto property set to true.
We are porting an existing excel chart application to .net with help
of a third party tool for charting.
If MajorUnitIsAuto property set to true, excel automatically
calculates the major unit.Need to know what algorithm excel uses to
calculate the Major Units.

Thanks in advance,
Mathew