How Chart Axis major unit Are Determined

[Gosha Sn@per]

Hello.I find article - http://support.microsoft.com/kb/214075/en-us. It
contains information on the calculation of the maximum and minimum values
​​on the axis, but does not contain information about major unit spacing.if
anyone knows the answer, I'll be very grateful....please help.
P.S.
major unit spacing can be equal 1*(10^n) or 2*(10^n) or 5*(10^n).
for example, max y = 36, min y = 27(min line = 30,max line = 35) ,major
unit spacing y= step = 1.
how they are step = 1, I do not understand...