Probably the most base solution is to make a sub-report displaying the chart only and exclude the Blank values in the record selection of the sub report.
If you do not have any duplicates and are doing counts (not distinctcounts) you could create a formula field to insert 1 or 0 (1!="", 0="") and then show the value as the SUM of that in the chart.
If table.field="" then 0 else 1
This would drop the "" grouping off the chart if you have it set to top N (10) and there are at least 10 other categories with at least 1 record.
You could use a Running Total but not as a top N. Set it to conditionally count records where field <> "".
Edited by DBlank - 08 Oct 2009 at 2:45pm