<Regular Axis> and <Irregular Axis>
A <Regular Axis> specifies an <Axis> defined for a numeric attribute (specified by the axis_type), where the tick marks specified are regularly spaced, starting at some specified first value.
An <Irregular Axis> is also defined for a numeric attribute, but the spacing of the tick marks is irregular, so the tick mark values are supplied by an array rather than specifying tick mark spacing.
For a more detailed treatment of <Axis> instances, attend Advanced Application of the DRM.