When several fields form a natural hierarchy, it makes sense to create a drill-down group. Typical examples of hierarchic groups are:
Time: Year, Quarter, Month
Geography: Continent, Country, State, City
When a drill-down group is used as a dimension in a chart, the chart will use the first fields 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.
In the first example above, Year will be used as chart dimension until a single year is selected. The chart will then show Quarter. If a single quarter is selected, the chart will switch to Month.
As selections disappear, so that more than one field becomes available in the upper fields of the group's field list, the chart will automatically be drilled back up. Forced drill-up can be achieved by clicking on the drill-up icon in the chart.
A special feature appears when the same drill-down group is used in more than one dimension in the chart specification. The second time the group occurs, the field used will automatically be taken from one step down the group's field list. For example, if you create a two-dimensional chart with the group Geography above used as both Main dimension and 2nd dimension, Continent and Country will initially be used. As soon as one single continent is selected, Country and State will be used instead.