long doRangeClassification(string attrName, int n)

This method classifies the layer (which must be a database layer) into n groups, based on attrName, where attrName is the name of a column in the corresponding database table.  This means that each feature in the layer is assigned to one of n groups, where the range of values in each group is the range of values in the designated column divided by n.  Each group is then assigned a distinct color.  For example, if the following holds:

then, there will be 4 distinct colors, classifying the cities into 4 groups according to population, as folllows: 1 to 3.5 million, 3.5 to 6 million, 6 to 8.5 million, and 8.5 to 11 million. 

The colors are assigned based on the current default color scheme, which is set using overrideDefaultColorScheme.

This method returns zero upon success, and returns an error code if: