When values from a dimension and a measure are placed in a table, data for null values will appear in the table without a label:

This lack of label will chart over into the legend of any table turned into a chart, so in order to label the row or column (i.e. “Not Answered” or “Unknown”), the following formula can be used to create a desired label:

=If(IsNull([Gender])) Then "Not Answered" Else [Gender]

Translation: If the value for gender is null/blank then insert the text in quotation marks (i.e. “Not Answered”).  Otherwise, insert the gender value (i.e. male or female)