From the manual ....
DRILL DOWN
When several fields form a natural hierarchy, it makes sense to create a drill-down
group. Typical examples of hierarchic groups could be:
Time:Year, Quarter, Month
or
Geography: Continent, Country, State, City
When a drill-down group is used as a dimension in a chart, the chart will use the first
field in the group's list of fields that has more than one possible value. If selections
are made that cause the field to have only one possible value, the next field in the list
will be used instead, provided that it has more than one possible value. If no field in
the list has more than one possible value, the last field will be used anyway.
Cyclic
Sometimes it may be useful to group also fields which do not form a natural hierarchy
or even have nothing in common at all. The reason would be to enable the user to
make quick changes of the data to be displayed in a chart.
Any fields can be grouped together in a cyclic group. When a cyclic group is
used as a dimension in a chart, the chart will initially use the first field in the
group's list of fields. The user may then switch to another field by clicking the
cycle icon in the chart. The fields are used in the order they appear in the group's field
list. When the last field in the list has been used, the turn goes back to the first field.
The chart may be cycled indefinitely.
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