When you create additional Member Columns and Action Tag Columns, you may also want to define those columns so that they are available as charts in Reports.
For example, let's say you add a column to your Members table called Country. By defining this column, you may see reports which break down your data by Country in the Demographic Charts section of each type of report. Once you've defined Country, for instance, you may go to Reports: Members: Demographic Charts and view member data as broken down by country.
Not all demographic information is useful to chart. For example, charting zip codes may be interesting, but charting social security numbers would probably not be (as the data is unique for each member).