Gets or sets the scale step size for this Scale (the increment between
labeled axis values).
The value is defined in user scale units
This value can be set
automatically based on the state of MajorStepAuto. If
this value is set manually, then MajorStepAuto will
also be set to false. This value is ignored for Log
axes. For Date axes, this
value is defined in units of MajorUnit.
